UC-NRLF 


DAIRY  BAC 


RUSSELL 


REESE  LIBRARY 

OF  THK 

UNIVERSITY  OF  CALIFORNIA 


,  190    . 
Accession  No.     92011     .   Class  No. 


\ 


OUTLINES 


OF 


DAIRY  BACTERIOLOGY 


A  CONCISE  MANUAL  FOR  THE  USE  OF 
STUDENTS  IN  DAIRYING 


BY 

H.  L,  RUSSELL 
*  / 

PROFESSOR  OF  BACTERIOLOGY,  UNIVERSITY  OF  WISCONSIN 


FIFTH  EDITION 

THOROUGHLY  REVISED 

UNIVERSITY 


MADISON,  WISCONSIN 

PUBLISHED  BY  THE  AUTHOR 

1902 


COPYRIGHTED  1903 

BY 
EL  L.  RUSSELL. 


STATE  JOURNAL  PRINTING  COMPANY, 

PRINTERS  AND  STEREOTYPERS, 

MADISON,  wis. 


PREFACE  TO  FIFTH  EDITION. 


Knowledge  in  claiming,  like  all  other  technical  indus- 
tries, has  grown  mainly  out  of  experience.  Many  facts 
have  been  learned  by  observation,  but  the  why  of  each  is 
frequently  shrouded  in  mystery. 

Modern  dairying  is  attempting  to  build  its  more  accu- 
rate knowledge  upon  a  broader  and  surer  foundation,  and 
in  doing  this  is  seeking  to  ascertain  the  cause  of  well- 
established  processes.  In  this,  bacteriology  is  playing  an 
important  r61e.  Indeed,  it  may  be  safely  predicted  that 
future  progress  in  dairying  will,  to  a  large  extent,  depend 
upon  bacteriological  research.  As  Fleischmanu,  the  emi- 
nent German  dairy  scientist,  says:  "The  gradual  abolition 
of  uncertainty  surrounding  dairy  manufacture  is  the  pres- 
ent important  duty  which  lies  before  us,  and  its  solution 
can  only  be  effected  by  bacteriology." 

It  is  therefore  natural  that  the  subject  of  Dairy  Bacteri- 
ology has  come  to  occupy  an  important  place  in  the  cur- 
riculum of  almost  every  Dairy  School.  An  exposition  of 
its  principles  is  now  recognized  as  an  integral  part  of 
dairy  science,  for  modern  dairy  practice  is  rapidly  adopt- 
ing the  methods  that  have  been  developed  as  the  result  of 
bacteriological  study.  The  rapid  development  of  the  sub- 
ject has  necessitated  a  frequent  revision  of  this  work,  and 
it  is  gratifying  to  the  writer  that  the  attempt  which  has 
been  made  to  keep  these  Outlines  abreast  of  bacteriological 
advance  has  been  appreciated  by  students  of  dairying. 

While  the  text  is  prepared  more  especially  for  the  prac- 

92011 


iv  Preface  to  Fifth  Edition. 

tical  dairy  operator  who  wishes  to  understand  the  principles 
and  reasons  underlying  his  art,  numerous  references  to  orig- 
inal investigations  have  been  added  to  aid  the  dairy  inves- 
tigator who  wishes  to  work  up  the  subject  more  thor- 
oughly. 

My  acknowledgments  are  due  to  the  following  for  the 
loan  of  illustrations:  Wisconsin  Agricultural  Experiment 
Station;  Creamery  Package  Mfg.  Co.,  Chicago,  111.;  and  A.  H. 
Reid,  Philadelphia,  Pa, 

H.  L.  RUSSELL. 
UNrvEitsiTY  OP  WISCONSIN, 
Madison,  January,  1902. 


CONTENTS. 


CHAPTER       1  Structure  of  the  bacteria  and  conditions  gov- 
erning their  development  and  distribution      1 

CHAPTER      II.  Methods  of  studying  bacteria 13 

CHAPTER     III.  Contamination  of  milk 19 

CHAPTER     IV.  Fermentations  in  milk  and  their  treatment    62 

CHAPTER       V.  Relation  of  disease -bacteria  to  milk 82 

Diseases  transmissible  from  animal  to  man 

through  diseased  milk 84 

Diseases  transmissible  to  man  through  in- 
fection of  milk  after  withdrawal 94 

CHAPTER     VI.  Preservation  of  milk  for  commercial  purposes  102 

CHAPTER    VIL  Bacteria  and  butter  making 134 

Bacterial  defects  in  butter 156 

CHAPTER  VI1L  Bacteria  in  cheese 160 

Influence  Of  bacteria  in  normal  cheese 

processes 160 

Influence  of  bacteria  in  abnormal  cheese 
processes 182 


CHAPTER  I. 

STRUCTURE  OF  THE  BACTERIA  AND   CON- 
DITIONS GOVERNING  THEIR  DEVELOP- 
MENT AND  DISTRIBUTION* 

BEFORE  one  can  gain  any  intelligent  conception  of  the 
manner  in  which  bacteria  affect  dairying,  it  is  first  neces- 
sary to  know  something  of  the  life  history  of  these  organ- 
isms in  general,  how  they  live,  move  and  react  toward  their 
environment. 

Nature  of  Bacteria.  Toadstools,  smuts,  rusts  and  mil- 
dews are  known  to  even  the  casual  observer,  because  they 
are  of  evident  size.  Their  plant-like  nature  can  be  more 
readily  understood  from  their  general  structure  and  habits 
of  life.  The  bacteria,  however,  are  so  small,  that  under  ordi- 
nary conditions,  they  only  become  evident  to  our  unaided 
senses  by  the  by-products  of  their  activity. 

When  Leeuwenhoek  (pronounced  Lave-en-hake)  in  1675 
first  discovered  these  tiny,  rapidly-moving  organisms  he 
thought  they  were  animals.  Indeed,  under  a  microscope, 
many  of  them  bear  a  close  resemblance  to  those  minute 
worms  found  in  vinegar  that  are  known  as  "  vinegar-eels." 
The  idea  that  they  belonged  to  the  animal  kingdom  con- 
tinued to  hold  ground  until  after  the  middle  of  the  present 
century;  but  with  the  improvement  in  microscopes,  a  more 
thorough  study  of  these  tiny  structures  was  made  possible, 
and  their  vegetable  nature  demonstrated.  The  bacteria  as 
a  class  are  separated  from  the  fungi  mainly  by  their  method 
of  growth;  from  the  lower  algae  by  the  absence  of  chloro- 
phyll, the  green  coloring  matter  of  vegetable  organisms. 


2  Dairy  Bacteriology. 

Structure  Of  bacteria.  So  far  as  structure  is  concerned 
the  bacteria  stand  on  the  lowest  plane  of  vegetable  life. 
The  single  individual  is  composed  of  but,  a  single  cell,  the 
structure  of  which  does  not  differ  essentially  from  that  of 
many  of  the  higher  types  of  plant  life.  It  is  composed  of 
a  protoplasmic  body  which  is  surrounded  by  a  thin  mem- 
brane that  separates  it  from  neighboring  cells  that  are 
alike  in  form  and  size. 

Form  and  size.  When  a  plant  is  composed  of  a  single 
cell  but  little  difference  in  form  is  to  be  expected.  While 
there  are  intermediate  stages  that  grade  insensibly  into 
each  other,  the  bacteria  may  be  grouped  into  three  main 


r  *     i  » 


Fio.  1.  Different  forms  of  bacteria,  a,  b,  c,  represent  different  types  as  to 
form:  a,  coccus,  *,  bacillus,  c,  spirillum;  </,  diplococcus  or  twin  coccus  ;<?,  staphy- 
lococcus  or  cluster  coccus;  /  anl  #,  different  forms  of  bacilli,  g  shows  internal 
endospores  within  cell;  h  and  /,  bacilli  with  motile  organs  (cilia). 

types,  so  far  as  form  is  concerned.  These  are  spherical, 
elongated,  and  spiral,  and  to  these  different  types  are  given 
the  names,  respectively,  coccus,  bacillus  and  spirillum 
(plural,  cocci,  bacilli,  spirilla)  (fig.  1).  A  ball,  a  short 
rod,  and  a  corkscrew  serve  as  convenient  models  to  illus- 
trate these  different  forms. 

In  size,  the  bacteria  are  the  smallest  organisms  that  are 
known  to  exist.  Relatively  there  is  considerable  difference  in 


Structure  of  the  Bacteria.  3 

size  between  the  different  species,  yet  in  absolute  amount, 
this  is  so  slight  as  to  require  the  highest  powers  of  the 
microscope  to  detect  it.  As  an  average  diameter,  one  thirty- 
thousandth  of  an  inch  may  be  taken.  It  is  difficult  to 
comprehend  such  minute  measurements,  but  if  a  hundred 
individual  germs  could  be  placed  side  by  side,  their  total 
thickness  would  not  equal  that  of  a  single  sheet  of  paper 
•upon  which  this  page  is  printed. 

Manner  Of  Growth.  As  the  cell  increases  in  size  as  a 
result  of  growth,  it  elongates  in  one  direction,  and  finally 
a  new  cell  wall  is  formed,  dividing  the  so-called  mother- 
cell  into  two,  equal-sized  daughter-cells.  This  process  of 
cell  division,  known  as  fission,  is  continued  until  growth 
•ceases  and  is  especially  characteristic  of  bacteria. 

Cell  Arrangement.  If  fission  goes  on  in  the  same 
plane  continually,  it  results  in  the  formation  of  a  cell-row. 
A  coccus  forming  such  a  chain  of  cells  is  called  strepto- 
coccus (chain-coccus).  If  only  two  cells  cohere,  it  is  called  a 
diplo-coccus  (twin-coccus).  If  the  second  cell  division  plane  is 
formed  at  right  angles  to  the  first,  a  cell  surface  or  tetrad 
is  formed.  If  growth  takes  place  in  three  dimensions  of 
•space,  a  cell  mass  or  sarcina  is  produced.  Frequently,  these 
•cell  aggregates  cohere  so  tenaciously  that  this  arrangement 
is  of  value  in  distinguishing  different  species. 

Spores.  Some  bacteria  possess  the  property  of  forming 
spores  within  the  mother  cell  (called  endospores,  fig.  Ig) 
that  are  analogous  in  function  to  the  seeds  of  higher  plants 
and  spores  of  fungi.  By  means  of  these  structures  which 
are  endowed  with  greater  powers  of  resistance  than  the 
vegetating  cell,  the  organism  is  able  to  protect  itself  from 
the  effect  of  an  unfavorable  environment.  Many  of  the 
bacilli  form  endospores  but  the  cocci  do  not.  It  is  these 


4:  .      Dairy  Bacteriology. 

spore  forms  that  resist  the  action  of  heat  in  pasteurizing 
milk. 

Movement.  Many  bacteria  are  unable  to  move  from 
place  to  place.  They  have,  however,  a  vibrating  movement 
known  as  the  Brownian  motion  that  is  purely  physical. 
Many  other  kinds  are  endowed  with  powers  of  locomotion. 
Motion  is  produced  by  means  of  tine  thread-like  processes 
of  protoplasm  known  as  cilia  (sing,  cilium)  that  are  de- 
veloped on  the  outer  surface  of  the  cell.  By  means  of  the 
rapid  vibration  of  these  organs,  the  cell  is  propelled  through 
the  medium.  Nearly  all  cocci  are  im motile,  while  the 
bacilli  may  or  may  not  be.  These  cilia  are  so  delicate  that 
it  requires  special  treatment  to  demonstrate  their  presence. 

Classification.  In  classifying  or  arranging  the  different 
members  of  any  group  of  living  objects,  certain  similarities 
and  dissimilarities  must  be  considered.  These  are  usually 
those  that  pertain  to  the  structure  and  form,  as  such 
are  regarded  as  most  constant.  With  the  bacteria 
these  differences  are  so  slight  that  they  alone  do  not  suffice 
to  distinguish  distinctly  one  species  from  another.  As  far 
as  these  characters  can  be  used,  they  are  taken,  but  in 
addition,  many  characteristics  of  a  physiological  nature  are 
added.  The  wa}T  that  the  organism  grows  in  different  kinds 
of  cultures,  the  by-products  produced  in  different  media, 
and  effect  on  the  animal  body  when  injected  into  the  same 
are  also  used  as  data  in  distinguishing  one  species  from 
another. 

Conditions  favoring  bacterial  growth.  The  bacteria,  in 
common  with  all  other  living  organisms  are  affected  by 
external  conditions,  either  favorably  or  unfavorably.  Cer- 
tain conditions  must  prevail  before  development  can  occur. 
Thus,  the  organism  must  be  supplied  with  an  adequate 


Structure  of  the  Bacteria.  5 

and  suitable  food  supply  and  with  moisture.  The  tem- 
perature must  also  range  between  certain  limits,  and  finally, 
the  oxygen  requirements  of  the. organism  must  be  con- 
sidered. 

Food  supply.  Most  bacteria  are  capable  of  living  on 
dead,  inert,  organic. matter,  such  as  meats,  milk  and  vege- 
table material,  in  which  case,  they  are  known  as  sap- 
rophytes. In  contradistinction  to  this  class  is  a  smaller 
group  known  as  parasites,  which  derive  their  nourishment 
from  the  living  tissues  of  animals  or  plants.  The  first 
group  comprise  by  far  the  larger  number  of  known  organ- 
isms which  are  concerned  for  the  most  part  in  the  decom- 
position of  organic  matter.  The  parasitic  group  includes 
those  which  are  the  cause  of  various  communicable  diseases. 
Between  these  two  groups  there  is  no  sharp  line  of  division, 
and  in  some  cases,  certain  species  possess  the  faculty  of 
growing  either  as  parasites  or  saprophytes,  in  which  case 
they  are  known  as  facultative  parasites  or  saprophytes. 

The  great  majority  of  bacteria  of  interest  in  dairying 
belong  to  the  saprophytic  class ;  only  those  species 
capable  of  infecting  milk  through  the  development  of  dis- 
ease in  the  animal  are  parasites  in  the  strict  sense  of  the 
term.  Most  disease-producing  species,  as  diphtheria  or 
typhoid  fever,  while  parasitic  in  man  lead  a  saprophytic 
method  of  life  so  far  as  their  relation  to  milk  is  concerned. 

Bacteria  require  for  their  growth,  nitrogen,  hydrogen, 
carbon,  oxygen,  together  with  a  limited  amount  of 
mineral  matter.  The  nitrogen  and  carbon  are  most  avail- 
able in  the  form  of  organic  compounds,  such  as  albuminous 
material.  Carbon  in  the  form  of  carbohydrates,  as  sugar 
or  starch,  is  most  readily  attacked  by  bacteria. 

Inasmuch  as  the  bacteria  are  plant-cells,  they  must  im- 


6  Dairy  Bacteriology. 

bibe  their  food  from  material  in  solution.  They  are  capable 
of  living  on  solid  substances,  but  in  such  cases,  the  food 
elements  must  be  rendered  soluble,  before  they  can  be  ap- 
propriated. If  nutritive  liquids  are  too  highly  concen- 
trated, as  in  the  case  of  syrups  and  condensed  milk,  bacteria 
cannot  grow  therein,  although  all  the  necessary  ingredients 
may  be  present.  Generally,  bacteria  prefer  a  neutral  or 
slightly  alkaline  medium,  rather  than  one  of  acid  reaction; 
but  there  are  numerous  exceptions  to  this  general  rule, 
especially  among  the  bacteria  found  in  milk. 

Temperature.  Growth  of  bacteria  can  only  occur  with- 
in certain  temperature  limits,  the  extremes  of  which  are 
designated  as  the  minimum  and  maximum.  Below  and 
above  these  respective  b'mits,  life  may  be  retained  in  the 
cell  for  a  time,  but  actual  cell-multiplication  is  stopped. 
Somewhere  between  these  two  cardinal  temperature  points, 
and  generally  nearer  the  maximum  limit  is  the  most  favor- 
able temperature  for  growth,  known  as  the  optimum.  The 
temperature  zone  of  most  dairy  bacteria  in  which  growth 
occurs  ranges  from  40°-45°  F.  to  somewhat  above  blood- 
heat,  105°-110°  F.,  the  optimum  being  from  80°-95°  F. 
Many  parasitic  species,  because  of  their  adaptation  to  the 
bodies  of  warm-blooded  animals,  generally  have  a  narrower 
range,  and  a  higher  optimum,  usually  approxima'ing  the 
blood  heat  (9S°-99°  F).  The  broader  growth  limits  of  bac- 
teria in  comparison  with  other  kinds  of  life  explain  why 
these  organisms  are  so  widely  distributed  in  nature. 

Air  supply.  No  living  organism,  can  thrive  without 
oxygen  and  most  species  require  atmospheric  oxygen;  but 
there  exist  certain  bacteria  (and  yeasts  as  well),  which 
possess  the  peculiar  property  of  not  being  able  to  utilize 
elemental  oxygen.  These  secure  the  necessary  element  for 


Structure  of  the  Bacteria.  7 

respiratory  purposes  from  oxygen  in  combination,  and  on 
account  of  their  ability  to  thrive  in  an  atmosphere  devoid 
of  free  oxygen  are  called  anaerobic,  while  those  requiring 
air  are  called  aerobic.  Some  species  grow  strictly  under 
one  condition  or  the  other,  and  hence  are  obligate  aerobes 
or  anaerobes;  others  known  as  facultative  or  optional  forms 
possess  the  ability  of  growing  under  either  condition.  The 
majority  of  milk  bacteria  are  either  obligate  or  facultative 
aerobes. 

Rate  Of  Growth.  Growth  takes  place  very  slowly  at  or 
near  the  minimum  temperatures,  but  as  the  temperature 
is  increased  the  rate  of  growth  becomes  more  rapid,  until 
at  the  optimum  point,  a  single  bacterial  cell,  in  an  active 
vegetative  condition  may  divide  into  two  cells  in  twenty 
minutes.  If  the  temperature  is  raised  to  the  maximum  the 
rate  of  development  is  very  rapidly  lessened,  until  finally 
cell-multiplication  ceases  altogether.  Even  under  opti- 
mum conditions  this  initially  rapid  rate  of  development 
cannot  be  maintained  indefinitely,  for  growth  is  soon 
limited  by  the  accumulation  of  by-products  of  cell  activity. 
Thus,  the  sour  milk  germ  grows  rapidly  at  ordinary  tem- 
peratures until  the  accumulation  of  lactic  acid  checks  its 
own  growth.  If  this  is  removed  by  neutralizing  it  with 
chalk,  the  lactic  bacteria  renew  their  growth. 

Detrimental  effect  of  external  conditions.  Enviromental 
influences  of  a  detrimental  character  are  constantly  at  work 
on  bacteria,  tending  to  repress  their  development  or  destroy 
them.  These  act  much  more  readily  on  the  vegetating  cell 
than  on  the  more  resistant  spore.  A  thorough  knowl- 
edge of  the  effect  of  these  antagonistic  forces  is  essential, 
for  it  is  often  by  their  means  that  undesirable  bacteria  may 
be  killed  out. 


8  Dairy  Bacteriology. 

Effect  Of  cold.  While  it  is  true  that  chilling  largely  pre- 
vents fermentative  action,  and  actual  freezing  stops  all 
growth  processes,  still  it  does  not  follow  that  exposure  to 
low  temperatures  will  effectually  destroy  the  vitality  of 
bacteria,  even  in  the  vegetative  condition.  Numerous  non- 
spore-bearing  species  remain  alive  in  ice  for  a  prolonged 
period,  and  recent  experiments  with  liquid  air  show  that 
even  a  temperature  of  -310°  F.  for  hours  does  not  effect- 
ually kill  all  exposed  cells. 

Effect  of  heat.  High  temperatures,  on  the  other  hand, 
will  destroy  any  form  of  life,  whether  in  the  vegetative  or 
latent  stage.  The  temperature  at  which  the  vitality  of  the 
cell  is  lost  is  known  as  the  thermal  death  point.  This  limit 
is  not  only  dependent  upon  the  nature  of  the  organism,  but 
varies  with  the  time  of  exposure  and  the  condition  in 
which  the  heat  is  applied.  In  a  moist  atmosphere  the 
penetrating  power  of  heat  is  great;  consequently  cell-death 
occurs  at  a  lower  temperature  than  in  a  dry  atmosphere. 
An  increase  in  time  of  exposure  lowers  the  temperature 
point  at  which  death  occurs. 

For  vegetating  forms  the  thermal  death  point  of  most 
bacteria  ranges  from  130°-140°  F.  where  the  exposure  is 
made  for  ten  minutes  which  is  the  standard  arbitrarily 
selected.  In  the  spore  stage  resistance  is  greatly  increased, 
some  forms  being  able  to  withstand  steam  at  210°-212°  F. 
from  one  to  three  hours.  If  dry  heat  is  employed,  260°- 
300°  F.  for  an  hour  is  necessary  to  kill  spores.  Where  steam 
is  confined  under  pressure,  a  temperature  of  230°-240°  F. 
for  15-20  minutes  suffices  to  kill  all  spores. 

Drying.  Spore-bearing  bacteria  like  anthrax  withstand 
drying  with  impunity;  even  tuberculous  material,  although 
not  possessing  spores  retains  its  infectious  properties  for 


Structure  of  the  Bacteria.  9 

many  months.  Most  of  the  dairy  bacteria  do  not  produce 
spores,  and  yet  in  a  dry  condition,  they  retain  their  vitalit}?- 
unimpaired  for  considerable  periods,  if  they  are  not  sub- 
jected to  other  detrimental  influences. 

Light.  Bright  sunlight  exerts  on  many  species  a  power- 
ful disinfecting  action,  a  few  hours  being  sufficient  to  des- 
troy all  cells  that  are  reached  by  the  sun's  rays.  Even 
diffused  light  has  a  similar  effect,  although  naturally  less 
marked.  The  active  rays  in  this  disinfecting  action  are 
those  of  the  chemical  or  violet  end  of  the  spectrum,  and 
not  the  heat  or  red  rays. 

Influence  of  chemical  substances.  A  great  many  chem- 
ical substances  exert  a  more  or  less  powerful  toxic  action 
of  various  kinds  of  life.  Many  of  these  are  of  great  ser- 
vice in  destroying  or  holding  bacterial  growth  in  check. 
Those  that  are  toxic  and  result  in  the  death  of  the  cell  are 
known  as  disinfectants;  those  that  merely  inhibit,  or  re- 
tard growth  are  known  as  antiseptics.  All  disinfectants 
must  of  necessity  be  antiseptic  in  their  action,  but  not  all 
antiseptics  are  disinfectants  even  when  used  in  strong 
doses.  Disinfectants  have  no  place  in  dairy  work,  except 
to  destroy  disease  bacteria,  or  preserve  milk  for  analytical 
purposes.  Corrosive  sublimate  or  potassium  bichromate 
are  most  frequently  used  for  these  purposes.  The  so-called 
chemical  preservatives  used  to  "keep"  milk  depend  for 
their  effect  on  the  inhibition  of  bacterial  growth.  With  a 
substance  so  violently  toxic  as  formaldehyde  (known  as 
formalin,  freezene)  antiseptic  doses  are  likely  to  be  ex- 
ceeded. In  this  country  most  states  prohibit  the  use  of 
these  substances  in  milk.  Their  only  function  in  the  dairy 
should  be  to  check  fermentative  or  putrefactive  processes 
and  so  keep  the  air  free  from  taints. 


10  Dairy  Bacteriology. 

Products  of  growth.  All  bacteria  in  their  development 
form  certain  more  or  less  characteristic  by-products.  With 
most  dairy  bacteria,  these  products  are  formed  from  the 
decomposition  of  the  medium  in  which  the  bacteria  may 
happen  to  live.  Such  changes  are  known,  collectively,  as 
fermentations,  and  are  characterised  by  the  production  of 
a  large  amount  of  by-products,  as  a  result  of  the  develop- 
ment of  a  relatively  small  amount  of  cell-life.  The  souring 
of  milk,  the  formation  of  butyric  acid,  the  making  of  vin- 
egar from  cider,  are  all  examples  of  fermentative  changes. 

With  many  bacteria,  especially  those  that  affect  proteid 
matter,  foul-smelling  gases  are  formed.  These  are  known 
as  putrefactive  changes.  All  organic  matter,  under  the 
action  of  various  organisms,  sooner  or  later  undergoes 
decay,  and  in  different  stages  of  these  processes,  acids,  al- 
kalies, gases  and  numerous  other  products  are  formed. 
Many  of  these  changes  in  organic  matter  occur  only 
when  such  material  is  brought  in  direct  contact  with  the 
living  bacterial  cell. 

In  other  instances,  soluble,  non-vital  ferments  known  as 
enzyms  are  produced  by  the  living  cell,  which  are  able  to 
act  on  organic  matter,  in  a  medium  free  from  live  cells,  or 
under  conditions  where  the  activity  of  the  cell  is  wholly 
suspended.  These  enzyms  are  not  confined  to  bacteria 
but  are  found  throughout  the  animal  and  plant  world, 
especially  in  those  processes  that  are  concerned  in  diges- 
tion. Among  the  better  known  of  these  non-vital  fer- 
ments are  rennet,  the  milk-curdling  enzym;  diastase  or 
ptyalin  of  the  saliva,  the  starch-converting  enzym;  pepsin 
and  trypsiu,  the  digestive  ferments  of  the  animal  body. 

Enzyms  of  these  types  ar3  frequently  found  among  the 
bacteria  and  yeasts  and  it  is  by  virtue  of  this  characteristic 


Structure  of  the  Bacteria.  11 

that  these  organisms  are  able  to  break  clown  such  enor- 
mous quantities  of  organic  matter.  Most  of  these  en- 
zyms  react  toward  heat,  cold  and  chemical  poisons  in  a 
manner  quite  similar  to  the  living  cells.  In  one  respect 
they  are  readily  differentiated,  and  that  is,  that  practically 
all  of  them  are  capable  of  producing  their  characteristic 
chemical  transformations  under  anaesthetic  conditions,  as 
in  a  saturated  ether  or  chloroform  atmosphere. 

Distribution  Of  bacteria.  As  bacteria  possess  greater 
powers  of  resistance  than  most  other  forms  of  life,  they  are 
to  be  found  more  widely  distributed  than  any  other  type. 
At  the  surface  of  the  earth,  where  conditions  permit  of 
their  growth,  they  are  found  everywhere,  except  in  the 
healthy  tissues  of  animals  and  plants.  In  the  superficial 
soil  layers,  they  exist  in  myriads,  as  here  they  have  abund- 
ance of  nourishment.  At  the  depth  of  several  feet  how- 
ever, they. diminish  rapidly  in  numbers,  and  in  the  deeper 
soil  layers,  from  six  to  ten  feet  or  moi\),  they  are  not 
present,  because  of  the  unsuitable  growth  conditions. 

The  bacteria  are  found  in  the  the  air  because  of  their  de- 
velopment in  the  soil  below.  They  are  unable  to  grow 
even  in  a  moist  atmosphere,  but  are  so  readily  dislodged 
by  wind  currents  that  over  land  areas  the  lower  strata  of 
the  air  always  contain  them.  They  are  more  numerous 
in  summer  than  in  winter;  city  air  contains  larger  num- 
bers than  country  air.  Wherever  dried  fecal  matter  is 
present,  as  in  barns,  the  air  contains  many  forms. 

Water  contains  generally  enough  organic  matter  in  so- 
lution, so  that  certain  types  of  bacterial  life  find  favorable 
growth  conditions.  Water  in  contact  with  the  soil  surface 
takes  up  many  impurities,  and  is  of  necessity  rich  in  mi- 
crobes. As  the  rain  water  percolates  into  the  soil,  it  loses 


12  Dairy  Bacteriology. 

its  germ  content,  so  that  the  normal  ground  water,  like 
the  deeper  soil  layers,  contains  practically  no  bacterial  life. 
Springs  therefore  are  relatively  deficient  in  germ  life,  ex- 
cept as  they  become  infected  with  soil  organisms,  as  the 
water  issues  from  the  soil.  Water  may  serve  to  dissem- 
inate certain  infectious  diseases  as  typhoid  fever  and  cholera 
among  human  beings,  and  a  number  of  animal  maladies. 

While  the  inner  tissues  of  healthy  animals  are  free  from 
bacteria,  the  natural  passages  as  the  respiratory  and  digest- 
ive tracts,  baing  in  more  direct  contact  with  the  exterior,  be- 
come more  readily  infected.  This  is  particularly  true  with 
reference  to  the  intestinal  tract,  for  in  the  undigested 
residue,  bacterial  activity  is  at  a  maximum.  The  result  is 
that  fecal  matter  contains  enormous  numbers  of  organ- 
isms so  that  the  possibility  of  pollution  of  any  food  medium 
such  as  milk  with  such  material  is  sure  to  introduce  ele- 
ments that  seriously  affect  the  quality  of  the  product. 


CHAPTER  II. 
METHODS  OF  STUDYING  BACTERIA. 

Necessity  of  bacterial  masses  for  study.  The  bacteria 
are  so  extremely  small  that  it  is  impossible  to  study  indi- 
vidual germs  separately  without  the  aid  of  first-class  micro- 
scopes. For  this  reason,  but  little  advance  was  made  in 
the  knowledge  of  these  lower  forms  of  plant  life,  until  the 
introduction  of  culture  methods,  whereby  a  single  organ- 
ism could  be  cultivated  and  the  progeny  of  this  cell  in- 
creased to  such  an  extent  in  a  short  course  of  time,  that 
they  would  be  visible  to  the  unaided  eye. 

This  is  done  by  growing  the  bacteria  in  masses  on  vari- 
ous kinds  of  food  media  that  are  prepared  for  the  purpose, 
but  inasmuch  as  bacteria  are  so  universally  distributed,  it 
becomes  an  impossibility  to  cultivate  any  special  form, 
unless  the  medium  in  which  they  are  grown  is  first  freed 
from,  all  pre-existing  forms  of  germ  life.  So  accomplish 
this,  it  is  necessary  to  subject  the  nutrient  medium  to 
some  method  of  sterilization,  such  as  heat  or  filtration, 
whereby  all  life  is  completely  destroyed  or  eliminated. 
Such  material  after  it  has  been  rendered  germ-free  is  kept 
in  sterilized  glass  tubes  and  flasks,  and  is  protected  from 
infection  by  cotton  stoppers. 

Culture  media.  For  culture  media,  many  different 
substances  are  employed.  In  fact,  bacteria  will  grow  on 
almost  any  organic  substance  whether  it  is  solid  or  fluid, 
provided  the  other  essential  conditions  of  growth  are  fur- 
nished. The  food  substances  that  are  used  for  culture 
purposes  are  divided  into  two  classes;  solids  and  liquids. 


14:  Dairy  Bacteriology. 

Solid  media  may  be  either  permanently  solid  like  pota- 
toes, or  they  may  retain  their  solid  properties  only  at  cer- 
tain temperatures  like  gelatin  or  agar.  The  latter  two  are 
of  utmost  importance  in  bacteriological  research,  for  their 
use,  which  was  introduced  by  Koch,  permits  the  separation 
of  the  different  forms  that  may  happen  to  be  in  any  mix- 
ture. Gelatin  is  used  advantageously  because  the  majority 
of  bacteria  present  wider  differences  due  to  growth  upon 
this  medium  than  upon  any  other.  It  remains  solid  at 
ordinary  temperatures,  bacoming  liquid  at  about  70°  F. 
Agar,  a  gelatinous  product  derived  from  a  Japanese  sea- 
weed, has  a  much  higher  melting  point,  and  can  be  suc- 
cessfully used,  especially  with  those  organisms  whose 
optimum  growth  point  is  above  the  melting  point  of  gel- 
atin. 

Besides  these  solid  media,  different  liquid  substances  are 
extensively  used,  such  as  beef  broth,  milk,  and  infusions  of 
various  vegetable  and  animal  tissues.  Skim-milk  is  of 
especial  value  in  studying  the  milk  bacteria  and  may  be 
used  in  its  natural  condition,  or  a  few  drops  of  litmus  solu- 
tion may  be  added  in  order  to  detect  any  change  in  its 
chemical  reaction  due  to  the  bacteria. 

Methods  of  isolation.  Suppose  for  instance  one  wishes 
to  isolate  the  different  varieties  of  bacteria  found  in  milk. 
The  method  of  procedure  is  as  follows  :  Sterile  gelatin  in 
glass  tubes  is  melted  and  cooled  down  so  as  to  be  barely 
warm.  To  this  gelatin  which  is  germ-free  a  drop  of  milk 
is  added.  The  gelatin  is  then  gently  shaken  so  as  to  thor- 
oughly distribute  the  milk  particles,  and  poured  out  into  a 
sterile  flat  glass  dish  and  quickly  covered.  This  is  allowed 
to  stand  on  a  cool  surface  until  the  gelatin  hardens.  After 
the  culture  plate  has  been  left  for  twenty-four  to  thirty- 


Methods  of  Studying  Bacteria. 


15 


six  hours  at  the  proper  temperature,  tiny  spots  will  begin 
to  appear  on  the  surface,  or  in  the  depth  of  the  culture 
medium.  These  patches  are  called  colonies  and  are  com- 
posed of  an  almost  infinite  number  of  individual  germs, 
the  result  of  the  continued  growth  of  a  single  organism 


Fia.  2.  A  gelatin  plate  culture  showing  appearance  of  different  organisms 
in  a  sample  of  milk.  Each  mass  represents  a  bacterial  growth  (colony)  derived 
from  a  single  cell.  Different  forms  react  differently  toward  the  gelatin,  some 
liquefying  the  same,  others  growing  in  a  restricted  mass,  a,  represents  a  colony 
of  the  ordinary  bread  mold;  b,  a  liquefying  bacterium;  c,  and  dt  solid  forms. 

that  was  in  the  drop  of  milk  which  was  firmly  held  in 
place  when  the  gelatin  solidified.  The  number  of  these 
colonies  represents  approximately  the  number  of  germs 
that  were  present  in  the  milk  drop.  If  the  plate  is  not 
too  thickly  sown  with  these  germs,  the  colonies  will  con- 
tinue to  grow  and  increase  in  size,  and  as  they  do,  minute 
differences  will  begin  to  appear.  These  differences  may  be 
in  the  color,  the  contour  and  the  texture  of  the  colony,  or 


16 


Dairy  Bacteriology. 


the  manner  in  which  it  acts  toward  gelatin.  In  order  to 
make  sure  that  the  seeding  in  not  too  copious  so  as  to  in- 
terfere with  continued  study,  an  attenuation  is  usually 
made.  This  consists  in  taking  a  drop  of  the  infected  gel- 


Fio.  8.  Profile  view  of  gelatin  plate  culture ;  £,  a  liquefying  form  that  dissolves 
the  gelatin;  c  and  </,  surface  colonies  that  do  not  liquefy  the  gelatin. 

atin  in  the  first  tube,  and  transferring  it  to  another  tube 
of  sterile  media.  Usually  this  operation  is  repeated  again 
so  that  these  culture  plates  are  made  with  different 
amounts  of  seed  with  the  expectation  that  in  at  least  one 
plate  the  seeding  will  not  be  so  thick  as  to  prevent  further 
stady.  For  transferring  the  culture  a  loop  made  of  plat- 
inum wire  is  used.  By  passing  this  through  a  gas  flame,  it 
can  be  sufficiently  sterilized. 

To  further  study  the  peculiarities  of  different  germs,  the 
separate  colonies  are  transferred  to  other  sterile  tubes  of 
culture  material  and  thus  pure  cultures  of  the  various 
germs  are  secured.  These  cultures  then  serve  as  a  basis  for 
continued  study  and  must  be  planted  and  grown  upon  all 
the  different  kinds  of  media  that  are  obtainable.  In  this 
way  the  slight  variations  in  the  growth  of  different  forms 
are  detected  and  the  peculiar  characteristics  are  determined, 
so  that  the  student  is  able  to  recognize  this  form  when  he 
meets  it  again. 

These  culture  methods  are  of  essential  importance  in 
bacteriology,  as  it  is  the  only  way  in  which  it  is  possible 
to  secure  a  quantity  of  germs  of  the  same  kind. 


Methods  of  Studying  Bacteria. 


17 


The  microscope  in  bacterial  investigation.  In  order  to 
verify  the  purity  of  the  cultures,  the  microscope  is  in  con- 
stant demand  throughout  all  the  different  stages  of  the 


cl 


Fra  4.  Pure  cultures  of  different  kinds  of  bacteria  in  gelatin  tubes,  a,  growth 
slight  in  this  medium;  b,  growth  copious  at  and  near  surface.  Fine  parallel  fil- 
aments growing  out  into  medium  liquefying  at  surface;  c,  a  rapid  liquefying 
form;  d,  a  gas-producing  form  that  grows  equally  well  in  lower  part  of  tube  as 
at  surface  (facultative  anaerobe) ;  *,  an  obligate  anaerobe,  that  develops  only  in 
absence  of  air. 

isolating  process.     For  this  purpose,  it  is  essential  that  the 
instrument  used  shall  be  one  of  strong  magnifying  powers 
(600-800  diameters),  combined  with  sharp  definition. 
The  microscopical  examination  of  any  germ  is  quite  as 
2 


18  Dairy  Bacteriology. 

essential  as  the  determination  of  culture  characteristics; 
in  fact,  the  two  must  go  hand  in  hand.  The  examination 
reveals  not  only  the  form  and  si::e  of  the  individual  germs, 
but  the  manner  in  which  they  are  unite  1  with  each  other, 
as  well  as  any  peculiarities  of  movement  that  they  may 
possess. 

In  carrying  out  the  microscopical  part  of  the  work,  not 
only  is  the  organism  examined  in  a  living  condition,  but 
preparations  are  made  by  using  solutions  of  auilin  dyes  as 
staining  agents.  These  are  of  great  service  in  bringing 
out  almost  imperceptible  differences.  The  art  of  staining 
has  been  carried  to  the  highest  degree  of  perfection  in 
bacteriology,  especially  in  the  detection  of  germs  that  are 
found  in  diseased  tissues  in  the  animal  or  human  body. 

In  studying  the  peculiarities  of  any  special  organism, 
not  only  is  it  necessary  that  these  cultural  and  micro- 
scopical characters  should  be  closely  observed,  but  special 
experiments  must  ba  carried  out  along  different  lines,  in 
order  to  determine  any  special  properties  that  the  germ 
may  possess.  Thus,  the  ability  of  any  form  to  act  as  a 
fermentative  organism  can  be  tested  by  fermentation  ex- 
periments; the  property  of  causing  disease,  studied  by  the 
inoculation  of  pure  cultures  into  animals.  A  great  many 
different  methods  have  been  devised  for  the  purpose  of 
studying  special  characteristics  of  different  bacteria,  but  a 
full  description  of  these  would  necessarily  be  so  lengthy 
that  in  a  work  of  this  character  they  must  be  omitted. 
For  details  of  this  nature  consult  standard  reference  books 
on  bacteriological  technique. 


CHAPTER  III. 
CONTAMINATION  OF  MILK* 

Milk  as  food  for  bacteria.  The  fact  that  milk  undergoes 
decomposition  changes  so  readily  shows  that  it  is  well 
suited  to  nourish  bacterial  life.  Not  only  does  it  contain 
all  the  substances  necessary  for  nutrition,  but  they  are 
diluted  in  such  proportions  as  to  render  most  of  them  avail- 
able for  bacterial  as  well  as  mammalian  lite. 

The  albumen  which  is  in  solution  is  readily  assimilable. 
The  casein  can  not  be  appropriated  directly,  until  it  is  first 
rendered  soluble,  a  process  which  occurs  with  those  bac- 
teria that  secrete  proteid-dissolving  enzyms  like  pepsin 
and  trypsin.  Of  the  carbon-containing  compounds,  the 
fat  is  of  little  value  for  food  as  normally  bacteria  can  not 
act  on  this.  The  milk-sugar,  however,  is  an  admirable 
food  and  the  decomposition  of  this  results  in  the  produc- 
tion of  acids  and  gases.  The  mineral  requirements  of  the 
bacterial  cell  are  so  limited  as  to  demand  but  little  atten- 
tion, as  any  organic  substance  contains  sufficient  inorganic 
matter  to  satisfy  the  needs  of  the  cell  in  the  formation  of 
new  cell  substance. 

Milk,  germ-free  in  Udder.  Under  ordinary  conditions, 
when  examined  in  the  proper  manner,  milk  always  reveals 
bacterial  life.  This  germ  contend,  however,  is  due  to  in- 
fection from  without,  for  in  the  udder  of  a  healthy  animal, 
as  secreted,  the  milk  like  the  other  secretions  and  tissues 
of  the  body  is  normally  sterile. 

Contamination  Of  milk.     In  withdrawing  the  milk  from 

the  udder,  it  invariably  comes  in  contact  with  germ  life. 


20 


Dairy  Bacteriology. 


The  same  is  true  after  it  is  milked.  From  the  time  of  milk- 
ing, until  it  is  consumed  in  one  form  or  another,  it  is  con- 
tinually subject  to  contamination  from  external  sources. 
In  the  main,  germ  life  gains  access  while  the  milk  is  on  the 
farm,  but  even  in  the  factory,  the  opportunities  for  infec- 
tion are  present  in 
greater  or  lesser  de- 
gree. Those  forms 
that  become  estab- 
lished early  generally 
predominate  in  the 
milk,  as  their  intro- 
duction enables  them 
to  develop  for  a  longer  ^ 

period  of  time.  It  f 
must  be  remembered 
that  by  far  the  greater  Sb& 
part  of  these  organisms 
are  relatively  harmless. 
While  they  are  not 
concerned  in  the  production  of  disease,  the  larger  majority 
of  them  do  affect,  more  or  less  seriously,  the  quality  of 
the  milk.  They  are,  therefore,  undesirable  from  the  milk 
vendor's  point  of  view,  although  in  the  processes  of  butter 
and  theese-making,  the  presence  of  some  kinds  is  very  de- 
sirable. 

Under  the  varying  conditions  in  which  milk  is  handled 
it  must  of  necessity  be  infected  with  bacteria  of  different 
sorts,  yet  normally  the  type  of  fermentation  that  occurs  is 
quite  the  same,  showing  that  certain  species  find  milk  such 
a  favorable  nutrient  medium  that  they  soon  gain  the  ascend- 
ency over  other  forms. 


Fio.  5.    Microscopic  appearance  of  milk  show- 
ing relative  size  of  fat  globules  and  bacteria. 


Contamination  of  Milk.  21 

Sources  Of  infection.  The  bacterial  life  that  finds  its 
way  into  milk  while  it  is  yet  on  the  farm  may  be  traced  to 
several  sources,  which  may  be  grouped  under  the  following 
heads  :  unclean  dairy  utensils,  fore  milk,  coat  of  animal, 
and  general  atmospheric  surroundings.  The  relative  im- 
portance of  these  various  factors  fluctuates  in  each  individ- 
ual instance. 

Dairy  utensils.  Of  first  importance  are  the  vessels  that 
are  used  during  milking,  and  also  all  storage  cans  and 
other  dairy  utensils  that  come  in  contact  with  the  milk 
after  it  is  drawn.  By  unclean  utensils,  actually  visible 
dirt  need  not  always  be  considered,  although  its  presence 
in  cracks  and  corners  of  pails  and  cans  is  often  the 
case.  Unless  cleansed  with  special  care,  these  cracks  and 
joints  are  apt  to  be  filled  with  foul  and  decomposing  ma- 
terial that  suffices  to  abundantly  seed  the  milk.  Soxhlet1 
found  that  the  addition  of  0.1  per  cent  of  sour  milk  to 
fresh  milk  decreased  the  keeping  quality  of  the  latter  from 
15  to  30  per  cent ;  the  addition  of  1.5  per  cent  diminished  it 
80  per  cent.  Where  cans  are  not  well  cleansed  the  above 
amount  could  easily  be  added  to  the  milk  from  the  ma- 
terial that  adhered  to  the  walls  of  the  can. 

Through  negligence,  vessels  are  often  used  that  are  unfit 
for  handling  milk.     A  rusty  milk-can  often  spoils  more 
.  milk  than  sufficient  to  purchase  a  new  ves- 

\  /     sel.     Wooden  pails  are  no  longer  tolerated  in 

\       J^    a  well-regulated  dairy.     Where  possible,  ves- 
sels should  be  made  of  pressed  tin.      If  join's 
— £»—        are  necessary,  they  should  be  well  flushed  with 
FIG.  6.        solder  so  that  they  may  be  easily  and  thor- 
oughly cleaned   as  shown   in   Fig.   6.      In   much   of  the 

1  Soxhlet,  Ber.  d.  Wandervers.  bayer.  Landwirthe,  Oct.,  1894. 


22  Dairy  Bacteriology. 

cheaper  class  of  tinware  now  found  on  the  market,  the 
soldering  of  joints  and  seams  is  very  imperfect,  anJ.  this 
affords  a  place  of  refuge  for  bacteria  and  dirt,  as  shown  in 
c,  Fig.  6. 

Use  of  milk-cans  for  transporting  factory  by-products. 
The  general  custom  of  using  the  milk-cans  to  carry  back 
to  the  farm  the  factory  by-products  (skim-milk  or  whey) 
has  much  in  it  that  is  to  be  deprecated.  These  by-products 
are  generally  rich  in  bacterial  life,  more  especially  where 
the  closest  scrutiny  is  not  given  to  the  daily  cleaning  of 
the  vats  and  tanks.  Too  frequently  the  cans  are  not  cleaned 
immediately  upon  arrival  at  the  farm,  so  that  the  condi- 
tions are  favorable  for  rapid  fermentation.  Many  of  the 
taints  that  bother  factories  are  directly  traceable  to  such  a 
cause.  A  few  dirty  patrons  will  thus  seriously  infect  the 
whole  supply.  The  responsibility  for  this  defect  shouldt 
however,  not  be  laid  entirely  upon  the  shoulders  of  the 
producer.  The  factory  operator  should  see  that  the  refuse 
material  does  not  accumulate  in  the  waste  vats  from  day  to 
day  and  is  not  transformed  into  a  more  or  less  putrid  mass. 
A  dirty  whey  tank  is  not  an  especially  good  object  lesson 
to  the  patron  to  keep  his  cans  clean. 

It  is  possible  that  abnormal  fermentations  or  even  con- 
tagious diseases  may  thus  be  disseminated. 

Suppose  there  appears  in  a  dairy  an  infectious  milk 
trouble,  such  as  bitter  milk.  This  milk  is  taken  to  the 
factory  and  passes  unnoticed  into  the  general  milk-supply. 
The  skim-milk  from  the  separator  is  of  course  infected 
with  the  germ,  and  if  conditions  favor  its  growth,  the 
whole  lot  soon  becomes  tainted.  If  this  waste  product  is 
returned  to  the  different  patrons  in  the  same  cans  that  are 
used  for  the  fresh  milk,  the  probabilities  are  strongly  in 


Contamination  of  Mi 


favor  of  some  of  the  cans  being  contaminated  and  thus  in- 
fecting the  milk  supply  of  the  patrons.  If  the  organism 
is  endowed  with  spores  so  that  it  can  withstand  unfavor- 
able conditions,  this  taint  may  be  spread  from  patron  to 
patron  simply  through  the  infection  of  the  vessels  that  are 
used  in  the  transportation  of  the  by-products.  Connellhas 
reported  just  such  a  case  in  a  Canadian  cheese  factory  where 
an  outbreak  of  slimy  milk  was  traced  to  infected  whey  vats. 
Quite  a  number  of  epidemics  of  typhoid  fever  have  been 
shown  to  have  been  disseminated  in  this  way,  and  in  Den- 
mark and  Germany  with  foot  and  mouth  disease  and  tuber- 
culosis, the  danger  is  so  great  as  to  make  it  necessary  to 
heat  all  by-products  taken  back  to  the  farm.1 

Pollution  of  cans  from  whey  vats.  The  danger  is  greater 
in  cheese  factories  than  in  creameries,  for  whey  usually 
represents  a  more  advanced  stage  of  fermentation  than 
skim-milk.  The  higher  temperature  at  which  it  is  drawn 
facilitates  more  rapid  bacterial  growth,  and  the  conditions 
under  which  it  is  stored  in  many  factories  contribute  to  the 
ease  with  which  fermentative  changes  can  go  on  in  it. 
Often  this  by-product  is  stored  in  wooden  cisterns  or  tanks, 
situated  below  ground,  where  it  becomes  impossible  to 
clean  them  out  thoroughly.  A  custom  that  is  almost  uni- 
versally followed  in  the  Swiss  cheese  factories,  here  in  this 
country,  as  in  Switzerland,  is  fully  as  reprehensible  as  any 

1  Storch  (40  Kept.  Danish  Expt.  Stat.,  Copenhagen,  1898)  has  devised  a  test 
whereby  it  can  be  determined  whether  this  treatment  has  been  carried  out  or 
not:  Milk  contains  a  soluble  enzym  known  as  galactase  which  has  the  property 
of  decomposing  hydrogen  peroxid.  If  milk  is  heated  to  176°  F.  (80°  C.)  or  above, 
this  enzym  is  destroyed  so  that  the  above  reaction  w>  longer  takes  place.  If 
potassium  iodid  and  starch  are  added  to  unheated  milk  and  the  same  treated 
with  hydrogen  peroxid,  the  decomposition  of  the  latter  agent  releases  oxygen 
which  acts  on  the  potassium  salt,  which  in  turn  gives  off  free  iodine  that  turns 
the  starch  blue. 


24  Dairy  Bacteriology. 

dairy  custom  could  well  be.  In  Fig.  7  the  arrangement  in 
vogue  for  the  disposal  of  the  whey  is  shown.  The  hot 
whey  is  run  out  through  the  trough  from  the  factory  into 


FIG.  7.  Swiss  cheese  factory  (Wisconsin),  showing  careless  way  in  which  whey 
is  handled.  Each  patron's  share  is  placed  in  a  barrel,  from  which  it  is  removed 
by  him.  No  attempt  is  made  to  cleanse  these  receptacles. 

the  large  trough  that  is  placed  over  the  row  of  barrels,  as 
seen  in  the  foreground.  Each  patron  thus  has  allotted  to 
him  in  his  individual  barrel  his  portion  of  the  whey,  which 
he  is  supposed  to  remove  day  by  day.  No  attempt  is  made 
to  clean  out  these  receptacles,  and  the  inevitable  result  is 
that  they  become  filled  with  a  foul,  malodorous  liquid,  es- 
pecially in  summer.  When  such  material  is  taken  home 
in  the  same  set  of  cans  that  is  used  to  bring  the  fresh 
milk  (twice  a  day  as  is  the  usual  custom  in  Swiss  factories), 
it  is  no  wonder  that  this  industry  is  seriously  handicapped 
by  "  gassy  "  fermentations  that  injure  materially  the  qual- 
ity of  the  product.  Not  only  is  the  above  danger  a  very 


Contamination  of  Milk.        t  25 

considerable  one,  but  the  quality  of  the  factory  by-product 
for  feeding  purposes,  whether  it  is  skim-milk  or  whey,  is 
impaired  through  the  development  of  fermentative  changes. 

Improved  methods  of  disposal  of  by-products.  The  diffi- 
culties which  attend  the  distribution  of  these  factory  by- 
products have  led  to  different  methods  of  solution.  One  is 
to  use  another  separate  set  of  receptacles  to  carry  back 
these  products  to  the  farm.  This  method  has  been  tried, 
and  while  it  is  deemed  impracticable  by  many  to  handle  two 
sets  of  vessels,  yet  some  of  the  most  progressive  factories 
report  excellent  results  where  this  method  is  in  use. 

Large  barrels  could  be  used  for  this  purpose  to  econo- 
mize in  wagon  space. 

Another  method  that  has  met  with  wider  acceptance, 
especially  in  creameries,  is  the  custom  of  pasteurizing  or 
scalding  the  skim-milk  immediately  after  it  is  separated, 
so  that  it  is  returned  to  the  farmer  in  a  hot  condition.  In 
factories  where  the  whole  milk  is  pasteurized,  further 
treatment  of  the  by-product  is  not  necessary.  In  most 
factories  steam,  generally  exhaust,  is  used  directly  in  the 
milk,  and  experience  has  shown  that  such  milk,  without 
any  cooling,  will  keep  sweet  for  a  considerable  number  of 
hours  longer  than  the  untreated  product.  It  is  noteworthy 
that  the  most  advanced  and  progressive  factories  are  the 
ones  that  appreciate  the  value  of  this  work,  and  although 
it  involves  some  time  and  expense,  experience  has  shown 
the  utility  of  the  process  in  that  a  better  grade  of  milk  is 
furnished  by  the  patrons  of  factories  which  follow  this 
practice.1  The  exclusion  of  all  danger  of  animal  or  human 
disease  is  also  possible  in  this  way. 

i  McKay,  N.  Y.  Prod.  Rev..  Mch.  22, 1899. 


2G  Dairy  Bacteriology. 

Cleaning  dairy  utensils.  The  thorough  cleaning  of  all 
dairy  apparatus  that  in  any  way  conies  in  contact  with  the 
milk  is  one  of  the  most  fundamental  and  important  problems 
in  dairying.  All  such  apparatus  should  be  so  constructed 
as  to  permit  of  easy  cleaning.  Tinware,  preferably  of  the 
pressed  variety,  gives  the  best  surface  for  this  purpose  and 
is  best  suited  for  the  handling  of  milk. 

Milk  vessels  should  never  be  allowed  to  become  dry  when 
dirty,  for  dried  particles  of  milk  residue  are  extremely  diffi- 
cult to  remove.  In  cleaning  dairy  utensils  they  should 
first  be  rinsed  in  lukewarm  instead  of  hot  water,  so  as  to 
remove  organic  matter  without  coagulating  the  milk. 
Then  wash  thoroughly  in  hot  water,  using  soap  or  weak 
alkali.  A  borax  solution  is  sometimes  recommended  for 
cleaning  bottles.  Strong  alkalies  should  not  be  used. 
After  washing  rinse  thorough!}'  in  clean  hot  water.  If 
steam  is  available,  as  it  always  is  in  creameries,  cans  and 
pails  should  be  turned  over  jet  for  a  few  moments.  While 
a  momentary  exposure  will  not  suffice  to  completely  ster- 
ilize such  a  vessel,  yet  many  bacteria  are  killed  in  even  a 
short  exposure,  and  the  cans  dry  more  thoroughly  and 
quickly  when  heated  by  steam. 

Not  only  should  the  greatest  care  be  paid  to  the  condi- 
tion of  the  cans  and  milk-pails,  but  all  dippers,  strainers, 
and  other  utensils  that  come  in  contact  with  the  milk 
must  be  kept  equally  clean.  Cloth  strainers,  unless  attended 
to,  are  objectionable,  for  the  fine  mesh  of  the  cloth  retains 
so  much  moisture  that  they  become  a  veritable  hot-bed  of 
bacterial  life,  unless  they  are  daily  boiled  or  steamed. 

Germ  content  of  milk  utensils.  Naturally  the  number 
of  bacteria  found  in  different  milk  utensils  after  they  have 
received  their  regular  cleaning  will  be  subject  to  great 


Contamination  of  Milk.  27 

fluctuations;  but,  nevertheless,  such  determinations  are  of 
value  as  giving  a  scientific  foundation  for  practical  meth- 
ods of  improvement.  The  following  studies  may  serve  to 
indicate  the  relative  importance  of  the  utensils  as  a  factor 
in  milk  contamination. 

Two  cans  were  taken,  one  of  which  had  been  cleaned  in 
the  ordinary  way,  while  the  other  was  sterilized  by  steam- 
ing. Before,  milking,  the  udder  was  thoroughly  cleaned 
and  special  precautions  taken  to  avoid  raising  of  dust;  the 
fore  milk  was  rejected.  Milk  drawn  into  these  two  cans 
showed  the  following  germ  content: 

No.  bacteria     Hours  before 
per  cc.  souring. 

Steamed  pail 165  28£ 

Ordinary  pail 4265  23 

Harrison1  showed  the  great  variation  in  the  bacterial 
content  in  milk  cans  in  the  following  way:  Cans  were 
rinsed  with  100  cc.  of  sterile  water  and  numerical  deter- 
minations of  this  rinsing  water  made.  The  following  data 
are  from  cans  poorly  cleaned  (Series  A),  cans  washed  in 
tepid  water  and  then  scalded  —  the  usual  factory  method  — 
(•Series  B),  and  cans  washed  in  tepid  water  and  steamed  for 
five  minutes  (Series  C). 

Bacterial  contents  of  cans  cleaned  in  various  ways. 

Series  A,  dirty  cans 238,525  342,875  215,400  618,200  806,320 

510,270  230,100  610,510  418,810  317,250 

Series  B,  ordinary  method    89,320    84,750    26,800    24,000    38,400 

76,800    15,200    13,080    44,160    93,400 

Series  C,  approved  method      1,170      1,792         890         355         416 

A  variation  of  this  method  was  made  by  the  writer  as 
follows:  Three  pails  were  thoroughly  washed  with  100  cc. 

»  Harrison,  22  Kept.  Out.  Agr'l  Coll.,  1896,  p.  113. 


I. 

II. 

III. 

7,°00,000 

1,450,000 

49,000 

283,000 

43,400 

35,000 

1,685,000 

105,000 

61,400 

28  Dairy  Bacteriology. 

of  sterile  water,  using  a  sterile  swab  to  remove  all  dirt. 
This  process  was  repeated  with  two  otlrr  rinse  waters  of 
the  same  volume,  and  from  each  of  these  plates  were  made 
with  following  results: 

No.  bacteria  in  different  washings.  Total  No. 

bacteria. 
9,299,000 
361,400 
1,851,400 

Infection  of  udder  cavity.  While  it  may  be  true  that 
milk  as  secreted  in  the  glandular  tissue  of  a  healthy  ani- 
mal may  be  practically  iree  from  bacteria,  yet  it  does  not 
follow  that  it  contains  no  microbes  when  first  drawn.  In- 
deed, a  bacterial  examination  of  the  first  few  streams  taken 
from  each  teat  will  invariably  show  a  relatively  high  germ 
content;  much  higher,  in  fact,  than  that  which  is  subse- 
quently withdrawn.  The  reason  for  this  is  evident  when 
the  structure  of  the  udder  and  its  relation  to  the  exte- 
rior is  noted  (Fig.  8).  The  udder  is  composed  of  secreting 
tissue  (gland  cells),  held  in  place  by  fibrous  connective  tis- 
sue. Ramifying  throughout  this  glandular  structure  are 
numerous  channels  (milk  sinuses)  that  serve  to  convey  the 
milk  from  the  cells  where  it  is  produced  into  the  milk  cis- 
tern, a  common  receptacle  just  above  the  teat.  This  cavity 
is  connected  with  the  exterior  through  the  milk  duct  in  the 
teat,  which  is  closed  more  or  less  tightly  by  the  circular 
sphincter  muscles,  thus  preventing  the  milk  from  flowing 
out. 

According  to  the  best  authorities,  the  fat  globules  are 
elaborated  very  largely  during  the  actual  manipulation  of 
the  udder  in  milking,  yet  there  is  always  a  small  residual 
amount  of  milk  that  is  not  removed  even  by  "  clean"  milk- 
ers. 


Contamination  of  Milk. 


29 


The  ready  infection1  of  the  external  opening  of  the 
milk  duct  gives  an  opportunity  for  bacteria  to  plant  them- 
selves on  a  moist  mucous  surface,  and  the  result  is,  that 

some  organisms,  at 
least,  find  it  possi- 
ble to  penetrate  the 
milk  duct  and  so 
enter  the  milk  cis- 
tern. When  once 
this  reservoir  is  in- 
fected,  further 
spread  can  be  easily 
made,  even  into  the 
glandular  portion 
of  the  udder.2  In 
such  a  habitat,  ideal 
conditions  would 
seem  to  exist,  at 
least,  for  the  faculta- 
tive anaerobic  type 
of  bacteria.  Moist- 
ture,  warmth,  suffi- 
cient and  nutritious 
food,  give  optimum 
conditions  for  de- 
velopment, but 
these  may,  in  part 

FIG.  8.    Section  of  udder,  showing  teat,  milk  cis-      ,  ,        ,     , 
tern  and  secreting  tissue  (Moore  and  Ward).  at  least>  be 

by    the    fact 


that 


1  According  to  Jos.  Simon  (Diss.  Hyg.  Inst.  Erlangen,  1898)  the  udder  is  sterile 
except  at  outer  opening. 
8  Moore  and  Ward,  Bull.  153,  Cornell  Expt.  Stat,  Jan.,  1899. 


30  Dairy  Bacteriology. 

healthy  mucous  surfaces  secrete  more  or  less  marked  ger- 
micidal  fluids.1 

Number  of  bacteria  in  fore  milk.  If  the  first  few  streams 
of  milk  drawn  from  the  udder  are  examined  bacteriologic- 
ally,  it  will  invariably  be  found  that  the  same  contain  a 
relatively  large  number  of  organisms,  as  shown  by  the  fol- 
lowing data  collected  by  Harrison,2  in  which  the  bacterial 
content  of  the  fore  milk  is  compared  with  the  balance  of 
the  milking. 

Comparison  in  germ  content  of  fore  and  whole  milk. 

Foremilk 26,070  25,630  38,420  18,110  54,800    32,700 

43,520  27,830  18,500  29,400  45,630    48,700 
Milk  after  removal 

of  foremilk..'..     1,246  1,150  1,430  3,420  1,560         890 

2,575  4,820  3,270  1,285  1,350 

If  successive  bacterial  determinations  are  made  of  milk 
taken  at  different  periods  of  the  milking  process,  it  is  to  be 
noted,  as  in  the  following  experiment,  that  a  sudden  dimi- 
nution occurs  after  the  first  few  streams  are  removed.  In 
this  the  bacterial  distribution  per  cc.  was  as  follows: 

Bacterial  content  at  different  periods  of  milking. 

Fore      200th    2000th    4300th    6500th       Strip- 
milk,        cc.          cc.          cc.          cc.          pings. 

Expt.  1 6,500      1,700         475         220  75  5 

Expt.  2 3,100      1,650         400         240  50  10 

In  these  cases  contamination  from  all  other  sources  was 
excluded.  The  stoppings  will  sometimes  be  almost  sterile, 

^The  germicidal  properties  of  freshly  drawn  milk  discovered  by  Fokker 
(Zeit.  f.  Hyg.,  1890,  9:41)  is  shown  by  the  diminution  in  number  of  organisms  that 
may  sometimes  be  noted  (Park,  N.  Y.  Univ.  Bull.,  1901, 1:85). 

»  Harrison,  22  Kept.  Ont.  Agr.  Coll.,  1896,  p.  108;  also  Moore,  12  Kept.  Bur.  Animal 
Ind.  Washington,  1895-6,  p.  261. 


Contamination  of  Milk.  31 

thus  indicating  that  the  abundance  of  bacteria  found  in 
the  fore  milk  is  due  to  the  flushing  out  of  the  lower  por- 
tion of  the  cistern  and  duct  by  the  first  few  streams  re- 
moved. 

Kinds  Of  bacteria  in  fore  milk.  The  effect  of  these  or- 
ganisms on  milk  will  depend  upon  the  character  of  the 
same.  As  a  rule  the  number  of  the  different  species  found 
is  usually  small,  a  condition  due  in  all  probability  to  the 
fact  that  the  surroundings  in  the  udder  favor  a  rapid 
growth  of  certain  forms.  According  to  Boiley1  the  bac- 
terial flora  may  vary  considerably,  although  certain  types 
reappear  with  striking  constancy  if  once  found  in  the 
udder  or  teat.  What  these  forms  are  is  a  question  of  con- 
siderable importance,  for  it  would  seem  that  the  numerical 
predominance  of  those  present  in  the  fore  milk  might  in- 
fluence the  character  of  the  fermentation  of  the  whole 
milk. 

Harrison 2  claims  to  have  found  peptonizing  bacteria  in 
the  fore  milk,  while  Marshall 3  reports  organisms  that  re- 
sist pasteurizing.  Boiley,  in  thirty  experiments,  found 
twelve  out  of  sixteen  species  to  belong  to  the  lactic  class. 
Boiley  and  Hall  failed  to  find  gas-producing  forms  in  the 
milk  of  ten  cows  that  were  examined  for  a  period  of  three 
months;  but  the  observations  of  Moore  and  Ward4  show 
that  gas-generating  and  taint-producing  species  are  to  be 
found.  This  fact  is  important  in  the  selection  of  milk 
from  a  single  animal  for  the  cultivation  of  a  starter.  Hast- 
ings has  made  the  interesting  observation  in  the  writer's 
laboratory,  that  the  fore  milk,  although  much  richer  in 

i  Boiley,  Cent.  f.  Bakt.,  II  Abt,,  1895, 1:795. 

a  Harrison,  1.  c.,  p.  108. 

•Marshall,  Bull.  147,  Mich.  Expt.  Stat.,  p.  42. 

*  Moore  and  Ward,  Bull.  158,  Cornell  Expt.  Stat.,  Jan.,  1899. 


32  Dairy  Bacteriology. 

bacteria  than  the  whole  milk,  does  not  coagulate  nearly  as 
soon.  In  a  series  of  five  trials  the  fore  milk  did  not  curdle 
at  room  temperature  on  an  average  until  after  84  hours, 
while  the  average  time  of  curdling  of  the  whole  milk  was 
38  hours. 

Not  all  species  of  bacteria  seem  to  be  able  to  maintain 
themselves  in  the  udder  even  if  they  are  introduced  therein. 
Dinwiddie1  injected  into  the  milk  cistern  a  facultative  an- 
aerobic lactic  acid-producing  form  that  grew  at  99°  F. 
Several  subsequent  examinations  failed  to  reveal  the  or- 
ganism in  any  case.  Ward 2  experimented  with  B.  prodigi- 
osus  which  he  introduced  through  the  milk  duct.  He  was 
able  to  determine  its  presence  six  days  later.  Experiments 
have  also  been  made  with  B.  coli  communis,  B.  cloacae  and 
B.  lactis  aerogenes?  all  of  which  are  gas-generating  species, 
but  in  no  case  did  these  forms  thrive. 

In  securing  milk  under  conditions  whereby  the  bacterial 
content  is  reduced  as  much  as  possible,  it  is  advisable  to 
throw  out  these  first  few  streams.  In  doing  so,  the  in- 
trinsic loss  is  practically  negligible,  for  the  amount  of 
butter  fat  in  even  the  first  pint  of  a  milking  is  only  about 
0.7,4  or  one  fifth  of  the  normal. 

Infection  directly  from  animal.  It  is  a  popular  belief 
that  much  of  the  germ  life  that  is  found  in  milk  is  derived 
from  the  food  that  is  consumed  by  the  animal,  but  such  a 
condition  cannot  prevail  in  the  healthy  animal  for  the 
reason  that  bacteria  in  and  on  fodder  are  not  absorbed  into 
the  tissues  proper,  or  if  they  are,  they  are  quickly  killed 
by  the  germicidal  properties  of  the  body  fluids.  The 

»  Dinwiddie,  Bull.  45,  Ark.  Expt.  Stat.,  p.  57. 
'Ward,  Journ.  of  Appld.  Mic.,  1898,  1:305. 
»  Appel,  Milch  Zeit.,  1900,  No.  17. 
«  Snyder,  Chemistry  of  Dairying,  p.  10. 


Contamination  of  Milk.  33 

ger  which  does  obtain  directly  from  the  animal,  and  which 
to  some  extent  is  modified  by  the  nature  of  the  food  par- 
taken, is  due  to  the  fecal  matter  that  is  voided  from  the 
intestine.  Under  careless  conditions  this  is  permitted  to 
soil  the  flanks  and  udder  of  the  animal.  In  a  dried  state 
it  is  readily  dislodged  by  the  movements  of  milking  and  so 
falls  into  the  open  milk  pail.  The  nature  of  the  food  con- 
sumed modifies  to  some  extent  the  character  and  consist- 
ency of  the  manure,  physically  as  well  as  bacteriologically. 
The  modern  use  of  a  more  nitrogenous  ration  than  for- 
merly has  resulted  in  the  production  of  a  softer,  more  fluid 
manure,  which  is  more  likely  to  soil  the  coat  of  the  ani- 
mal. The  same  is  true  with  animals  on  pasture  in  com- 
parison with  those  fed  dry  fodder. 

Wiithrich  and  Freudenreich  *  find  a  markedly  higher 
germ  content  in  manure  where  animals  are  given  dry 
feed  than  where  kept  on  grass.  They  found  as  many  as 
375,000,000  bacteria  per  gram  in  fresh  manure,  the  major- 
ity of  which  consisted  of  B.  coli  communis,  the  hay  bacillus, 
and  other  species  able  to  peptonize  casein. 

Organisms  of  this  type  are  more  abundant  in  winter 
milk  than  in  summer,  as  the  opportunity  for  infection  is 
greatly  increased  by  closer  housing. 

The  nature  of  the  animal's  coat  favors  greatly  the  re- 
tention of  dirt  and  dust.  Cows  wading  in  slime-covered 
pools  may  cover  the  udder  with  material  teeming  with  bac- 
teria, which  falls  as  an  impalpable  powder  when  dry.  The 
danger  which  may  come  from  the  introduction  of  such 
matter  is  readily  seen  if  hairs  are  removed  from  the  coat 
of  the  animal  and  laid  on  the  surface  of  a  sterile  gelatin 
plate  as  in  Fig.  9.  Almost  invariably,  a  number  of  colo- 

'  Cent.  f.  Bakt.,  II.  Abt.   1895, 1:8?3. 

3 


34:  Dairy  Bacteriology. 

nies  develop  under  these  conditions,  thus  indicating  the  in- 
fection that  arises  from  this  material.  The  introduction 
of  the  dirt  particles  themselves,  however,  adds  relatively 
many  more  bac- 
teria than  come 
from  the  hairs 
themselves.  The 
amount  of  pol- 
lution coming 
from  the  coat  of 
the  animal  is 
largely  depend- 
ent upon  the 
care  taken  in 
bedding,  and 
even  the  nature 
of  the  bedding 
material  has  an 

effect.         Expen-  3^  9     showing  the  bacterial  contamination  arising 

ence    has    shown  from  hair.    These  hairs  were  allowed  to  fall  on  a  sterile 

1                       j.       •  gelatin   surface.      The   adherent  bacteria  developed 

P6at        3  readily  in  jjjjg  medium,  and  the  number  of  bacteria 

USed  for  this  pur-  thus  introduced  into  the  milk  from  these  hairs  can  be 

DOSe       that      the  estimated  by  the  number  of  developing  colonies. 

bacterial  life  is  greatly  reduced,  due  to  the  antiseptic  ac- 
tion which  this  strongly  acid  material  possesses. 

The  amount  of  impurities  that  are  often  to  be  found  in 
milk,  even  after  it  is  strained,  attest  beyond  dispute  the 
careless  methods  of  handling;  it  should,  furthermore,  be 
noted  that  about  one-half  of  fresh  manure  dissolves  in 
milk,1  and  thus  does  not  appear  as  sediment.  It  has  been 
determined  by  actual  tests  that  the  daily  milk  supply  of 

»  Backhaus,  Milch  Zeit,  1897,  2G:357. 


Contamination  of  Milk.  35 

the  city  of  Berlin,  Germany,  contains  about  three  hundred 
pounds  of  dirt  and  filth. 

From  a  large  number  of  determinations  of  the  solid  im- 
purities found  in  market  milk,  Renk !  deduces  the  follow- 
ing rule:  If  a  sample  of  milk  shows  any  evidence  of  im- 
purity settling  on  a  transparent  bottom  within  two  hours, 
it  is  to  be  regarded  as  containing  too  much  solid  impuri- 
ties. These  solid  particles,  composed  largely  of  manure 
and  dirt,  are  always  teeming  with  bacteria,  especially  with 
putrefactive  and  decomposition  organisms. 

While  the  number  of  bacteria  that  are  hereby  introduced 
into  milk  is  at  times  large,  the  character  of  the  species  is 
even  more  significant.  Derived  primarily,  as  most  of  them 
are,  from  fecal  matter  or  from  dirt,  it  is  little  wonder  that, 
their  introduction  should  call  forth  abnormal  fermenta- 
tions. Undoubtedly  bacteria  of  this  class  are  intimately 
concerned  in  the  production  of  intestinal  troubles  in  in- 
fants. Eckles2  has  shown  that  the  digesting  bacteria 
that  accompany  fecal  matter  are  closely  connected  with  the 
peculiar  winter  flavors  that  impair  the  quality  of  winter 
butter. 

Influence  of  the  milker.  The  condition  of  the  person 
of  the  milker  is  not  to  be  ignored  in  determining  all  pos- 
sible factors  of  infection,  for  when  clothed  in  dust-laden 
garments,  dislodgment  of  bacteria  takes  place  readily. 
Particular  attention  should  be  paid  to  the  hands  of  the 
milker.  The  filthy  practice  of  moistening  the  hands  with 
a  few  drops  of  milk  is  to  be  deprecated  from  every  point 
of  view.  The  milker  should  wash  his  hands  in  clean 
water  just  before  milking.  If  something  is  needed  to  en- 

»  Renk,  Cent.  f.  Bakt.,  1891,  10:19$. 

2  Eckles,  Bull.  59,  Iowa  Expt.  Stat.,  Aug.  1901. 


36  Dairy  Bacteriology. 

able  him  to  obtain  a  firmer  grasp,  a  pinch  of  vaseline  may 
be  used.  Any  scales  or  dirt  rubbed  from  the  teat  would 
be  held  by  the  vaseline  and  its  effect  on  sore  or  chapped 
teats  is  healing. 

Freudenreich  *  reports  some  experiments  in  which  the 
germ  content  of  milk  was  reduced  from  several  thousand 
to  200  where  the  hands  were  well  rubbed  with  vaseline 
before  milking.  Where  a  stringent  control  is  exercised,  it 
is  worth  while  to  have  the  milker  clothed  in  a  suit  kept 
for  this  purpose,  especially  the  upper  portion  of  the  body. 
An  outer  garment  could  easily  be  slipped  over  the  regular 
working  clothes.  This  garment  should  be  white  so  as  to 
necessitate  frequent  washing. 

Use  Of  milking  machines.  Numerous  attempts  have 
been  made  to  reduce  the  process  of  milking  to  a  mechan- 
ical basis  by  the  introduction  of  a  milking  machine  and 
with  some  of  these  devices  a  bacteriological  examination 
has  been  made.  Harrison 8  found  that  milk  drawn  from 
the  animal  with  the  Thistle  machine  was  much  richer  in 
bacteria  than  hand-drawn  milk.  This  was  due  to  the 
suction  applied  to  the  external  surface  of  the  teat  and 
udder,  which  caused  the  introduction  of  dust  particles.  In 
the  Murchland  machine,3  the  keeping  quality  of  the  milk 
was  fully  equal  to  that  drawn  in  the  usual  way. 

Exclusion  Of  dirt.  Scrupulous  care  will  greatly  mini- 
mize the  extent  of  infection  from  dirt  and  dust,  carding 
and  brushing  the  udder  and  flanks  will  remove  loose  hairs 
and  considerable  adherent  dirt,  but  so  long  as  the  coat  is 
dry,  dust  particles  and  bacteria  are  readily  dislodged.  It 
is  generally  thought  that  if  these  visible  evidences  of  dirt 

1  Freudenreich,  Die  Bakteriologie,  p.  80. 

a  Harrison,  Cent.  f.  Bakt.,  II  Abt.  1809,  5:183. 

•  Dyrsdale,  Trans.  High.  &  Agr.  Soc.  Scotland,  5  Sen,  1898, 10:166. 


Contamination  of  Milk.  37 

are  removed  by  straining  or  filtering  the  milk,  the  source 
of  trouble  is  eliminated.  But  in  this  operation  only  the 
visible  dirt  is  taken  out.  The  invisible,  and  by  far  the 
more  dangerous  material,  is  the  bacterial  life  that  thus  be- 
comes established,  there  to  grow  and  develop.  To  remove 
the  dirt  after  it  has  once  come  in  contact  with  the  milk 
only  lessens  the  difficulty,  but  does  not  overcome  it. 

1.  Moistening  the  udder.  If  alter  brushing  and  remov- 
ing the  loose  hairs  and  dirt  that  can  be  readily  dislodged 
in  the  milking,  the  udder  and  under  parts  are  thoroughly 
moistened  with  water,  the  fine  dust-like  particles  will  be 
held  in  place.  When  moistened  the  surface  does  not  want 
to  be  dripping  wet.  The  objection  has  been  raised  to 
washing  and  cleaning  the  udder  that  the  yield  of  milk  is 
reduced,  but  Eckles1  concludes  from  experiments  that 
when  the  animal  becomes  accustomed  to  the  treatment,  no 
noticeable  effect  is  produced  either  in  amount  of  milk  or 
butter-fat. 

The  effect  of  this  method  on  reducing  the  number  of 
bacteria  dislodged  is  apparent  from  the  following  test 
which  was  made  on  a  cow  kept  on  pasture  and  milked  out 
of  doors.  A  sterile  gelatin  plate  was  exposed  for  sixty 
seconds  under  the  belly  in  close  proximity  to  the  milk 
pail.  Then,  the  udder,  flank,  and  legs  of  the  cow  were  thor- 
oughly cleaned  with  water,  and  the  milking  resumed.  A 
second  plate  was  then  exposed  in  the  same  place  for  an 
equal  length  of  time;  a  control  exposure  being  made  at  a 
distance  of  ten  feet  from  the  animal  and  six  feet  from  the 
ground  to  ascertain  the  germ  content  of  the  surrounding 
air. 

From  this  experiment  the  following  instructive  data 

» Eckles,  Hoard's  Dairyman,  July  8,  1898. 


38  Dairy  Bacteriology. 

were  gathered.  Where  the  animal  was  milked  without 
any  special  precautions  being  taken,  there  were  3,250 
bacteria  j00r  minute  deposited  on  an  area  equal  to  the  ex- 
posed top  of  a  ten  inch  milk-pail.  Where  the  cow  received 
the  precautionary  treatment  as  suggested  above,  there 
were  only  115  bacteria  per  minute  deposited  on  the  same 
area.  In  the  control  plate  sixty-five  bacteria  were  found. 
This  indicates  that  a  large  number  of  organisms  from  the 
dry  coat  of  the  animal  can  be  kept  out  of  milk  if  such 
simple  precautions  as  these  are  carried  out.  A  consider- 
able number  of  other  observations  have  been  collected  in 
the  writer's  laboratory,  and  it  has  frequently  been  found, 
that  in  the  case  of  well  kept  herds,  the  germ  content  of 
the  milk  in  the  pail  is  increased  from  20,000-40,030  bacteria 
per  minute  during  the  milking  period  by  the  dislodg- 
ment  of  organisms  from  the  animal. 

2.  Diminishing  exposed  surface  of  pail.  Another  method 
of  excluding  the  dirt,  in  part  at  least,  is  to  use  a  pail  having 
a  less  exposed  surface.  There  are  several  different  types  of 
these  sanitary  or  hygienic  pails  that  are  ussd  more  or  less 
in  the  better  type  of  dairies. 

Eckles  reports  following  data  where  a  covered  pail  with 
a  small  opening  was  used  in  comparison  with  a  common 
open  pail.  43,200  bacteria  per  cc.  were  found  in  the 
milk  drawn  in  a  common  pail  as  against  3200  per  cc.  in 
covered  pail.  The  milk  soured  in  43  hours  in  the  first 
case;  64  hours  in  the  latter  instance.  A  series  of  experi- 
ments made  in  writer's  laboratory  by  Darrow  with  the 
two  sterilized  pails  shown  in  Fig.  10  were  as  follows: 

No.  bacteria  per  cc*  in  milk. 

PailA 125         91        110         40        170        80         40 

PailB..  535       240        160        115        230        45        170 


Contamination  of  Milk.  39 

Pail  A  is  provided  with  a  small  opening  in  a  removable 
top  (C)  to  the  pail.  This  top  piece  is  arranged  so  that  a  layer 
of  muslin  or  flannel  (a,  a')  can  be  stretched  over  opening. 
On  this  is  placed  a  layer  of  absorbent  cotton  (&),  which  in 
turn  is  covered  with  another  sheet  of  muslin. 

Pail  B  is  provided  with  a  removable  top  in  the  lower 
part  of  which  is  fitted  another  connection  .5,  that  is  also 
removable.  This  part  is  provided  with  a  cotton  filter  (a), 
the  milk  also  passing  upward  through  a  wire  strainer  (b). 


FIG.  10.    Sanitary  milk  pails  designed  to  diminish  introduction  of  hairs,  etc., 
into  milk. 

The  efficiency  of  either  of  these  pails  is  apparent  from 
the  data  presented. 

3.  Cleaning  milk  by  centrifugal  force.  Another  method 
that  is  in  quite  common  use  is  to  pass  the  milk  through 
separators.  This  not  only  removes  practically  all  sus- 
pended particles  of  foreign  matter,  as  dirt,  hairs,  epithelial 
cells  or  other  debris,1  but  eliminates  a  large  part  of  the 
bacteria  with  the  centrifuge  slime.  By  weighing  the 
skim  milk,  cream  and  slime  carefully,  and  then  determin- 
ing the  germ  content  of  each,  it  is  possible  to  compute  the 

1  Backhaus  (Milch  Ztg.,  1897,  26:358)  found  that  95.6  per  cent  of  impurities  were 
thus  removed. 


40  Dairy  Bacteriology. 

relative  distribution  of  bacteria.  Eckles  and  Barnes1  have 
reported  a  series  of  studies  of  this  sort  in  which  they  found 
from  37  to  56  per  cent  of  the  organisms  removed  from 
the  milk  by  centrifugal  force.  An  average  of  their  re- 
sults showed  about  29  per  cent  of  total  bacterial  life  in  the 
skim  milk,  24  per  cent  in  the  cream,  and  about  47  per 
cent  in  the  slime.  Quantitatively  the  bacterial  content  of 
the  slime  is  always  exceedingly  high,  ranging  from  tens  of 
millions  to  billions  of  bacteria  per  gram. 

It  is  surprising  that  the  elimination  of  snch  a  consider- 
able proportion  of  organisms  should  not  materially  en- 
hance the  keeping  quality  of  the  purified  milk.  In  the 
work  above  reported,  although  the  number  of  bacteria  was 
diminished  15  to  50  per  cent,  the  diminution  in  develop- 
ment of  acidity  in  twenty-four  hours  in  the  purified  sam- 
ples was  only  a  few  hundredths  of  one  per  cent. 

The  effect  of  filtration  through  sand,  gravel  and  other 
substances  is  the  same  as  when  the  milk  is  passed  through 
the  separator,  the  aim  in  all  cases  being  to  purify  or  free 
the  milk  from  solid  impurities  that  find  their  way  into  the 
milk  largely  from  the  animal  herself.  Comparative  experi- 
ments made  by  Backhaus  and  Cronheim2  on  various  filter- 
ing devices  show  that  cellulose  yields  the  best  results. 
Schuppan 8  found  the  supply  of  a  Copenhagen  company 
that  used  a  gravel  filter  reduced  38  to  48  per  cent. 

Influence  of  barn  air.  It  is  impossible  to  separate  the 
influence  of  the  air  entirely  from  that  of  the  animal,  as 
the  dust  particles  from  the  coat  of  the  animal  must  of 
necessity  pass  through  the  air. 

Germ  life  cannot  develop  in  the  air,  but  in  a  dried  con- 

>  Eckles  and  Barnes,  Bull.  59,  Iowa  Expt.  Stot.,  Aug.  1901. 
•Journ.  f.  Landw.,  1897,45:223. 
»  Cent.  f.  Bakt.,  1893,  13:557. 


Contamination  of  Milk.  41 

dition,  organisms  retain  their  vitality  for  long  periods  of 
time.  The  use  of  dry  fodder,  the  bedding  of  animals  with 
straw  adds  greatly  to  the  amount  of  dust  particles,  and 
consequently  to  the  germ  life  floating  in  the  air,  as  seen  in 
Fig.  11.  Taints  in  milk  have  frequently  been  traced  to 
infection  arising  from  this  source. 


FIG.  11.  Effect  of  contaminated  air.  The  number  of  spots  indicate  the  col- 
onies that  have  developed  from  the  bacteria  which  f ell  in  30  seconds  on  the  sur- 
face of  the  gelatin  plate  (3  inches  in  diameter).  This  exposure  was  made  at  time 
the  cows  were  fed. 

While  the  stable  cannot  be  entirely  freed  from  dust,  yet 
the  effect  of  this  factor  can  be  greatly  minimized  by  a  little 
forethought.  Feeding  before  milking  adds  materially  to 
the  germ  content  of  the  air,  if  feed  is  of  dry  character.  If 
moistened  and  given  during  milking,  the  same  objection 
does  not  inure.  The  following  data  collated  by  Harrison 
shows  number  of  bacteria  per  minute  deposited  in  a  12-inch 
pail.  In  Series  A,  the  exposure  was  made  during  bedding; 
in  B,  one  hour  after  this  operation. 


42  Dairy  Bacteriology. 

Influence  of  dusty  air  on  germ  life. 

Series  A 16,000  13,536  12,216  12,890  15,340 

19,200  23,400  27,342  42,750  18,730 

Series  B 483  610  820  715  1,880 

2,112  1,650  990  1,342  2,370 

These  results  indicate  that  the  bacteria  are  in  the  main 
attached  to  particles  of  considerable  weight,  as  they  settle 
readily  to  the  floor. 

It  has  long  been  observed  that  the  milk  of  stall-kept 
animals  does  not  keep  as  well  as  that  milked  out-of-doors. 
In  some  of  the  better  sanitary  dairies,  it  is  customary  ta 
have  a  milking  room,  the  walls  of  which  may  be  kept 
moist,  or  at  least  free  from  dust,  and  in  this  way  eliminate 
the  effect  of  air  infection. 

Relative  importance  of  foregoing:  factors.  It  is  exceed- 
ingly difficult  to  measure  the  relative  values  of  these  dif- 
ferent methods  of  infection  that  have  been  cited,  for  they 
are  subject  to  so  much  fluctuation.  Where  the  milk  is 
handled  without  any  special  care,  unclean  dairy  utensils 
and  dirt  from  the  animal  are  the  most  important  sources 
of  pollution,  not  only  with  reference  to  the  actual  number 
of  bacteria  introduced,  but  more  particularly  as  to  the  effect 
which  bacteria  of  this  class  exert  on  the  milk.  If,  how- 
ever, careful  supervision  is  given  to  the  carrying  out  of  ra- 
tional methods  of  cleanliness,  the  most  important  factor 
contributing  to  the  germ  content  is  often  the  fore  milk. 

Sanitary  or  hygienic  milk.  By  putting  into  practice  the 
various  suggestions  that  have  been  made  with  reference  to- 
diminishing  the  bacterial  content  of  milk,  it  is  possible  to 
greatly  reduce  the  number  of  organisms  found  therein, 
and  at  the  same  time  materially  improve  the  keeping  qual- 


Contamination  of  Milk.  43 

ity  of  the  milk.  Backhaus1  estimates  that  the  germ  life 
in  milk  can  be  easily  reduced  to  one-two  thousandth  of  its 
original  number  by  using  care  in  milking.  He  reports  a 
series  of  experiments  covering  two  years  in  which  milk 
was  secured  that  averaged  less  than  10,000  bacteria  per  cc., 
while  that  secured  under  ordinary  conditions  averaged  over 
500,000. 

Fig.  13  gives  an  illustration  as  to  what  care  in  milking 
will  do  in  the  way  of  eliminating  bacteria.  Fig.  12  shows 
a  gelatin  plate  seeded  with  the  same  quantity  of  milk  that 
was  used  in  making  the  culture  indicated  by  Fig.  13.  The 
first  plate  was  inoculated  with  milk  drawn  under  ordi- 
nary conditions,  the  germ  content  of  which  was  found  to 
be  15,500  bacteria  per  cc.,  while  the  sample  secured  under 
as  nearly  aseptic  conditions  as  possible  (Fig.  13)  contained 
only  330  organisms  in  the  same  volume. 

Within  recent  years  there  has  been  more  or  less  gener- 
ally introduced  into  many  cities,  the  custom  of  supplying 
high  grade  milk  that  has  been  handled  in  a  way  so  as  to 
diminish  its  germ  content  as  much  as  possible.  Milk  of 
this  character  is  frequently  known  as  usanitary,"uhygienic" 
or  "certified,"  the  last  term  being  used  in  connection  with 
a  certification  from  veterinary  authorities  or  boards  of 
health  as  to  the  freedom  of  animals  from  contagious  dis- 
ease. Frequently  a  numerical  bacterial  standard  is  exacted 
as  a  pre-requisite  to  the  recommendation  of  the  board  of 
examining  physicians.  Thus,  the  Pediatric  Society  of 
Philadelphia  requires  all  children's  milk  that  receives  its 
recommendation  to  have  not  more  than  10,000  bacteria 
per  cc.  Such  a  standard  has  its  value  in  the  scrupulous 
cleanliness  that  must  prevail  in  order  to  secure  these  re- 

1  Backhaus,  Ber.  Landw.  Inst.  Univ.  Konigsberg,  1897,  2:12. 


44  Dairy  Bacteriology. 

suits.     This  in  itself  is  practically  a  guarantee  of  the  ab- 
sence of  those  forms  liable  to  produce  trouble  in  children. 
De  Schweinitz1  has  made  a  series  of  examinations  ex- 
tending throughout  a  year  on  such  a  sanitary  dairy  in 


FIG.  19.  Bacterial  content  of  milk  handled  in  ordinary  way.  Each  spot  rep- 
resents a  colony  growing  on  gelatin  plate.  Compare  with  Fig.  13,  where  same 
quantity  of  milk  is  used  in  making  culture.  Over  15,000  bacteria  per  cc.  in  this 


Washington.  The  average  of  113  samples  was  6,485  or- 
ganisms per  cc.  The  following  figures  for  a  week  in  sum- 
mer are  taken  from  the  regular  analytical  records  of  a 
Philadelphia  firm  that  makes  a  specialty  of  children's  milk. 

July  ...............      1234  567 

No.  bacteria  per  cc..     525    1,050    125    2,450    1,250    475    2,100 

From  a  practical  point  of  view,  the  improvement  in 
quality  of  sanitary  milk,  in  comparison  with  the  ordinary 
product  is  seen  in  the  enhanced  keeping  quality.  During 

»  De  Schweinitz,  Nat.  Med.  Rev.,  Apr.  1899. 


Contamination  of  Milk.  45 

the  Paris  Exposition  in  1900,  milk  and  cream  from  several 
such  dairies  in  the  United  States  were  shipped  to  Paris, 
arriving  in  good  condition  after  15-18  days  transit.  When 
milk  has  been  handled  in  such  a  way  that  by  the  time  it 


Fio.  18.  Bacterial  content  of  milk  drawn  with  care.  Diminished  germ  con- 
tent is  shown  by  smaller  number  of  colonies  (330  bacteria  per  cc.).  Compare 
this  culture  with  that  shown  in  Fig.  12. 

reaches  the  consumer  it  contains  no  more  germ  life  than 
this,  it  is  evident  that  it  much  more  nearly  approximates 
the  condition  which  exists  in  the  udder  of  the  animal  than 
the  milk  ordinarily  sold  by  the  milk  dealer. 

Considerable  difference  of  opinion  has  existed  in  the 
minds  of  the  medical  profession  as  to  the  relative  merits  of 
such  sanitary  milk  in  comparison  with  pasteurized  or 
sterilized  milk  as  food  for  children.  While  it  can  gener- 
ally be  shown  that  properly  pasteurized  milk  will  contain 
less  germ  life  than  this  which  has  been  milked  and  handled 
under  careful  sanitary  conditions,  yet  the  fact  that  the 


4G  Dairy  Bacteriology. 

low  bacterial  content  is  secured  in  the  latter  case  by  the 
elimination  rather  than  the  destruction  of  bacteria  is  a 
point  in  its  favor.  Unquestionably  such  sanitary  milk  is 
less  changed  from  the  normal  secretion  of  the  cow  than 
that  which  has  been  subjected  to  heat  sufficiently  high  to 
•destroy  the  bacteria  in  the  same. 

It  may  not  be  practical  under  all  conditions  where  milk 
is  produced  to  put  into  operation  all  of  the  precautions 
tfcat  have  been  previously  referred  to.  But  there  can  be 
no  doubt  where  milk  is  to  be  consumed  as  milk,  that  the 
introduction  of  these  or  similar  methods  would  greatly 
improve  the  quality  of  the  product.  Even  where  milk  is 
utilized  in  the  factory  or  creamery,  it  is  quite  essential 
that  it  should  be  as  nearly  normal  as  possible,  for  in  but- 
ter and  cheese  making,  the  quality  of  the  product  is  di- 
rectly dependent  upon  the  character  of  the  raw  material. 

Dairymen  have  learned  many  lessons  in  the  severe  school 
of  experience,  but  it  is  earnestly  to  be  hoped  that  future 
conditions  will  not  be  summed  up  in  the  words  of  the 
eminent  German  dairy  scientist,  Prof.  Fleischmann,  when 
he  says  that  "all  the  results  of  scientific  investigation 
which  have  found  such  great  practical  application  in  the 
treatment  of  disease,  in  disinfection,  and  in  the  preserva- 
tion of  various  products,  are  almost  entirely  ignored  in 
milking." 

Effect  of  temperature  on  bacterial  growth.  After  milk 
is  once  seeded  with  bacterial  life,  no  one  factor  exerts  so 
potent  an  effect  as  temperature  upon  the  rate  of  growth. 

Although  different  species  vary  in  their  rate  of  develop- 
ment, yet  moderately  warm  temperatures  from  75°  to  90° 
F.,  encourage  rapid  growth.  Unless  the  milk  is  quickly 
deprived  of  its  original  heat,  the  rate  of  the  fermentative 


Contamination  of  Milk.  47 

changes  will  be  much  increased  as  is  shown  in  following 
results  obtained  by  Freudenreich.1 

No.  of  bacteria  per  cc.  in  milk  kept  at  different  temperatures. 

77°  F.  95°  F. 

5  hrs.  after  milking  < 10.000 30,000 

8    "        "        "         25,000 12,000,000 

12    u        "        "        46,000 35,283,000 

26    "        "        " 5,700,000 50,000,000 

PPOQEAY  OF  A  SI/iQLE  GERM 
I/I  12  HOURS,  I/I  MILR  ALLOWED 
TO  COOL  /iATURALLY. 


-f,**  I/S  MILfT  COOLED  WITH 

''HUCA 


COLD  WATER. 

Fro.  14.    Effect  of  cooling  milk  on  the  growth  of  bacteria. 

If  a  can  of  milk  is  allowed  to  cool  naturally,  it  will  take 
several  hours  before  it  reaches  the  temperature  of  the  sur- 
rounding air.  During  this  time  the  organisms  in  the  fore 
milk  are  continuing  their  rapid  growth,  while  those  forms 
which  come  from  dust,  and  are  presumably  in  a  latent 
state,  awake  from  their  lethargy  under  the  influence  of 
these  favorable  surroundings.  If  bacteria  once  gain  an 
entrance  and  begin  to  germinate,  a  considerably  lower 
temperature  is  required  to  successfully  check  development 
than  to  hold  latent  organisms,  like  spores,  in  a  condition 
where  germination  will  not  occur. 

To  hasten  this  lowering  of  temperature  artificial  cooling 
is  a  necessity.  With  good  well  water  having  a  tempera- 

1  Freudenreich,  Ann.  de  Microg.,  1890,  2:115. 


48  Dairy  Bacteriology. 

ture  of  48°-50°  F.,  it  is  possible  to  chill  milk  sufficiently  to 
keep  it.  Where  cold  water  is  not  available,  ice  water 
should  be  used.  In  the  production  of  the  best  quality  of 
milk  for  the  factory,  this  factor  of  early  and  thorough 
cooling  is  entitled  to  more  weight  than  even  the  matter  of 
extreme  care  in  milking. 

Mixing:  night  and  morning  milk.  Common  experience 
has  often  shown  when  old  milk  is  mixed  with  new,  that 
the  fermentative  changes  are  more  rapid  than  would  have 
been  the  case  if  the  two  milks  had  been  kept  apart.  This 
is  most  frequently  observed  when  the  night  milk  is  cooled 
down  and  mixed  with  the  warm  morning  milk.  This 
often  imperfectly  understood  phenomenon  rests  upon  the 
relation  of  bacterial  growth  to  temperature.  The  night 
milk  may  be  cooled  down  to  50°  F.,  but  by  the  next  morn- 
ing it  has  considerably  more  bacteria  than  the  freshly 
drawn  sample,  the  temperature  of  which  may  be  90°  F. 
Now,  if  these  two  milkings  are  mixed,  the  temperature  of 
the  whole  mass  will  be  raised  to  a  point  that  is  more  fa- 
vorable for  the  growth  of  all  of  the  contained  bacteria 
than  it  would  be  if  the  older  milk  was  kept  chilled. 

Number  of  bacteria  in  milk.  The  germ  content  of 
milk  varies  so  greatly  that  unless  the  conditions  are  all 
known,  it  is  impossible  to  foretell  what  may  be  found 
therein.  An  examination  of  milk  will  often  reveal  a  dif- 
ference in  numbers,  ranging  from  a  few  score  of  germs  to 
hundreds  of  millions  per  cc.  The  presence  of  such  a  vary- 
ing number  is  dependent  upon  certain  factors,  as  the  age 
of  the  milk,  the  care  taken  during  the  milking,  and  also 
the  way  in  which  it  has  been  handled  since  that  time. 
Disregarding  milk  of  different  ages,  the  number  of  germs 
present  in  any  sample  bears  a  general  relation  to  the 


Contamination  of  Milk.  49 

amount  of  dirt  and  filth  with  which  it  has  come  in  contact 
since  it  was  drawn  from  the  cow.  Bacteria  and  filth  of  all 
kinds  are  so  intimately  associated  with  each  other  that  the 
presence  of  one  rightly  presupposes  that  of  the  other. 

As  to  the  numerical  content  of  any  milk,  there  is  such 
a  wide  variation  under  different  conditions  that  figures  are 
not  of  much  worth  unless  surrounding  conditions  are 
considered.  No  exact  relation  can  be  maintained  between 
the  number  of  bacteria  in  milk  and  the  development  of 
fermentative  products. 

Under  American  conditions  data  are  gradually  being 
accumulated,  but  the  subject  has  not  been  exhaustively 
studied.  Milk  in  this  country  as  it  reaches  the  consumer 
usually  contains  fewer  bacteria  than  are  to  be  found  in 
European  supplies,  although  as  Conn  has  pointed  out,  it  is 
often  older.  As  he  intimates  this  is  explained  by  the  rela- 
tively free  use  of  ice  in  this  country.  A  few  determina- 
tions of  the  bacterial  content  of  European  milks  that 
have  been  analyzed  biologically  will  illustrate  this  point. 

Renk1  found  in  Halle  milk  supply  6  to  30,000,000  germs 
per  cc.;  Cnopf 2  in  Munich  milk  supply  200,000  to  6,000,000 
per  cc.;  Uhl8  in  Giessen  milk  83,000  to  170,000,000  per  cc.; 
Clauss 4  in  Wurzburg  222,000  to  23,000,000  per  cc.;  Bujwid 
in  Warsaw  an  average  of  4,000,000  per  cc.,  and  Knochen- 
steirn 5  in  Dorpat  25,000,000  per  cc. 

Sedgwick  and  Batchelder 6  report  fifty-seven  samples  of 
Boston  milk  as  containing  from  30,000  to  4,220,000  per  cc. 
In  the  country,  they  found  in  the  milk  fresh  from  the  cow 

J  Renk,  Cent.  f.  Bakt.  10:193. 
8  Cnopf,  Ibid.  6:  553. 
»  Uhl,  Zeit.  f.  Hyg.,  1892,  12:475. 
4  Clauss,  Diss.  Wurzburg,  1889. 

*  Knochensteirn,  Chem.  Cent.,  11:62. 

•  Sedgwick  and  Batchelder,  Boston  Med.  Surg.  Journ.,  Jan.  14,  1892. 

4 


50  Dairy  Bacteriology. 

30,000,  and  in  the  milk  as  used  on  the  table  about  70,000 
organisms  per  cc.  Loveland  and  Watson '  found  in  the 
supply  of  Middletown,  Conn.,  from  11,000  to  85,500,000 
per  cc. 

Leighton8  studied  seventeen  dairies  at  Montclair,  N.  J., 
for  a  three-year  period  with  the  following  results  : 

Class  I,  dairies  averaging  below  15,000  bacteria  per  cc.; 

Class  II,  those  averaging  from  40,000  to  70,000  per  cc.; 
•  Class  III,  those  above  180,000. 

In  Class  I,  the  dairies  were  found  to  be  clean  and  well 
improved;  in  Class  II  the  conditions  were  as  satisfactory 
as  possible  with  the  crude  appliances  used;  Class  III  was 
as  a  whole  careless.  McDonnell3  sampled  352  lots  from 
eleven  American  cities.  The  worst  samples  were  found  in 
restaurants  and  with  small  retail  dealers.  28  per  cent 
of  all  samples  contained  less  than  100,000  bacteria  per  cc. 
while  34  per  cent  had  less  than  500,000.  Park 4  finds  in 
New  York  City  that  the  milk  in  the  shops  where  it  is 
generally  sold,  averages  during  the  coldest  weather  about 
250,000  organisms  per  cc.,  during  cool  weather  about 
1,000,000,  and  in  hot  weather  about  5,000,000  per  cc.  Eckles 6 
has  studied  the  flora  of  milk  under  factory  conditions. 
He  finds  from  one  to  five  million  organisms  per  cc.  in 
winter,  but  in  summer  there  may  be  from  fifteen  to 
thirty  millions. 

Bacterial  vs.  other  standards.  As  the  germ  content 
of  milk  is  subject  to  such  wide  variations,  it  is  practi- 
cally impossible  to  establish  a  numerical  standard,  al- 

1  Loveland  and  Watson,  Ch.  7  Rep.  Storrs  Stat.  (Conn.),  1894,  p.  72. 

»  Leighton,  Science,  Mar.  23,  1900. 

1  McDonnell,  Penn.  Dept.  Agr.  Kept.,  1897,  p.  561. 

«Park,  N.  Y.  Univ.  Bull.,  1901,  1:85. 

•  Eckles,  Bull.  59,  Iowa  Expt.  Stat.,  Aug.,  1901. 


Contamination  of  Milk.  51 

though  Bitter  states  that  50,000  organisms  per  cc.  should 
be  a  maximum  limit  in  milk  intended  as  human  food. 
As  a  result  of  a  study  of  the  supply  of  New  York  city, 
Park  believes  it  is  possible  for  a  milk  producer,  without 
adding  materially  to  his  expense,  to  secure  milk  which 
when  first  drawn  will  not  contain  on  an  average  more 
than  30,000  bacteria  in  hot  weather  and  25,000  in  cold 
weather.  If  such  milk  is  chilled  immediately  to  50°  F.,  it 
will  not  contain  more  than  100,000  per  cc.  in  twenty-four 
hours.  Rochester,  New  York,  has  already  tried  the  en- 
forcement of  a  standard  (100,000  per  cc.)  with  good  re- 
sults it  is  claimed.  The  practical  difficulties  to  contend 
with  in  establishing  a  milk  standard  based  upon  a  quanti- 
tative bacterial  determination  are  such  as  to  render  its  gen- 
eral adoption  extremely  problematical. 

Acid  test.  It  would  seem  that  some  other  test  which 
can  be  more  easily  employed,  even  though  it  might  not  be 
susceptible  to  an  equal  degree  of  accuracy,  would  be  pref- 
erable. Such  a  test  is  to  be  found  in  the  acid  test  which 
measures  the  acidity  of  the  milk  in  question.  There  are 
of  course  organisms  to  be  found  in  milk  that  do  not  pro- 
duce acid,  and  therefore  it  might  be  thought  that  the 
presence  of  such  would  militate  against  the  accuracy  of 
.such  a  test;  but  under  normal  conditions,  the  lactic  acid- 
producing  organisms  find  in  milk  so  congenial  a  sub- 
stratum for  their  development,  that  in  the  great  majority 
of  cases,  the  determination  of  the  acid  indicates  the  man- 
ner in  which  milk  has  been  handled.  High  acidity  is  either 
due  to  old  milk  (long  period  of  incubation)  or  insufficient 
cooling  (rapid  incubation).  Either  of  these  conditions 
permits  of  the  accumulation  of  bacterial  life,  and  therefore 
impairs  the  quality  of  the  milk.  The  determination  of  the 


52 


Dairy  Bacteriology. 


acid  in  milk  can  be  made  accurately  by  titration  with 
standard  solutions  of  alkali,  or  more  readily,  in  general 
work,  by  employing  the  Farrington  alkaline  tablet,  which 
is  a  combination  of  a  definite  amount  of  standard  alkali 
and  an  indicator  (phenolphthalein).  It  is  possible  by  the 
use  of  the  tablet  solution  to  make  as  accurate  determina- 
tion of  acidity  as  with  the  usual  standard  solutions  em- 
ployed in  the  laboratory,  but  the  method  may  also  be 
used  rapidly  in  such  a  way  as  to  give  an  approximate  deter- 
mination of  acid.  This  modification  is  very  serviceable  at 
the  weigh-can  or  intake  where  it  is  necessary  to  pass  judg- 
ment very  quickly  on  the  quality  of  the  milk. 


Fia.  15.    Apparatus  used  in  making  rapid  acid  test. 

Making  the  acid  test  Fig.  15  gives  the  apparatus  nec- 
essary for  this  rapid  testing  of  the  acidity  of  the  milk.  A 
solution  of  the  alkaline  tablets  is  first  prepared  by  dissolv- 


Contamination  of  Milk.  53 

ing  them  in  clean  soft  water,  one  tablet  for  each  ounce  of 
water;  thus  eight  tablets  to  an  eight-ounce  bottle  of  water. 
In  determining  the  acidity,  a  number  of  common  white  cups 
are  used,  one  for  each  patron.  Two  measures  full '  of  the 
alkali  solution  are  placed  in  each  cup,  and  then  as  the  milk 
is  received  at  the  weigh-can,  one  measure  of  milk  is  added 
to  the  alkali  solution  in  the  cup,2  and  the  whole  gently,  but 
thoroughly  shaken.  If  the  pink  color  of  the  alkali  solution 
persists  even  fainthr,  it  shows  that  there  is  not  enough  acid 
in  the  milk  to  neutralize  the  the  same;  if  it  disappears  alto- 
gether, leaving  the  milk  white  in  color,  it  indicates  that 
there  is  more  acid  in  the  milk  than  can  be  neutralized  by 
the  alkali  of  the  tablet  solution.  When  these  proportions 
of  milk  and  alkali  solution  (2:1)  are  employed,  it  indicates 
an  acidity  of  0.2  per  cent,  figured  as  lactic  acid.  Milk 
should  not  contain  more  than  this  amount  of  acid,  and 
under  good  methods  of  handling,  the  acidity  should  be 
brought  down  to  0.15  per  cent  if  possible. 

Circumstances  may  arise  that  might  lead  to  an  error  if 
this  method  is  blindly  followed.  It  has  been  pointed  out  * 
that  if  milk  is  allowed  to  stand  in  rusty  cans  for  some 
time  its  acid  content  is  diminished  materially.  The  aver- 
age acidity  of  nine  samples  of  milk  brought  to  a  factory  in 
clean  cans  was  .228  per  cent  acidity,  while  that  of  nine 
other  patrons  brought  in  rusty  cans  was  only  .134  per  cent. 
Milk  kept  in  rusty  cans  is  sure  to  contain  large  quantities 
of  bacteria,  even  though  its  acid  content  may  be  low. 

Kinds  Of  bacteria  in  milk.  The  number  of  bacteria  in 
milk  is  not  of  so  much  consequence  as  the  kind  present. 

1 A  brass  cartridge  shell  provided  with  a  handle  serves  admirably  for  a  meas- 
ure. 

2  Farrington,  Bull.  52,  Wis.  Expt.  Stat. 
8  Biddick,  Hoard's  Dairyman,  July  30, 1897. 


54  Dairy  Bacteriology. 

With  reference  to  the  number  of  kinds  present,  the  more  dirt 
and  foreign  matter  the  milk  contains,  the  larger  the  num- 
ber of  varieties  found  in  the  same.  While  milk  may  con- 
tain forms  that  are  injurious  to  man,  still  the  great  majority 
of  them  have  no  apparent  effect  on  human  health.  In 
their  effect  on  milk,  the  case  is  much  different.  Depend- 
ing upon  their  action  in  milk,  they  may  be  grouped  into 
three  classes: 

1.  Bacteria  that  exert  no  appreciable  effect  in  milk. 

2.  Bacteria  that  are  beneficial  by  reason  of  the  products 
which  they  form. 

3.  Bacteria  that  are  injurious  on  account  of  the  effect 
which  they  produce  in  milk. 

A  surprisingly  large  number  of  bacteria  that  are  found 
in  milk  belong  to  the  first  class.  Undoubtedly  they  af- 
fect the  chemical  characteristics  of  the  milk  somewhat, 
but  not  to  the  extent  that  it  becomes  physically  percep- 
tible. Eckles l  reports  in  a  creamery  supply  from  20  to  55 
per  cent  of  entire  flora  as  included  in  this  class. 

Those  species  that  are  concerned  in  the  production  of 
proper  flavor  and  aroma  in  butter,  and  which  are  also 
cdncerned  in  the  development  of  acid  and  possibly  asso- 
ciated with  formation  of  cheese  flavor  represent  the  sec- 
ond type.  Many  of  these  organisms  are  lactic  acid-pro- 
ducing, but  in  addition  to  these,  some  of  the  casein 
ferments  are  also  associated  with  aroma  production  in  but- 
ter. Under  normal  conditions  b}r  far  the  larger  proportion 
of  bacteria  present  in  milk  belong  to  the  lactic  acid  type. 
There  are  always  present,  though,  digesting  species  that 
are  able  to  grow  if  the  lactic  acid  forms  are  killed,  as  in 
pasteurizing. 

i  Eckles,  Bull.  59,  Iowa  Expt.  Stat.,  Aug.,  1901. 


Contamination  of  Milk.  55 

The  third  class  includes  those  species  that  are  able  to 
produce  deleterious  and  undesirable  flavors  in  milk  and 
milk  products.  The  abnormal  fermentations  of  milk  re- 
ferred to  in  the  next  chapter  come  under  this  head.  Most 
of  these  gain  access  to  the  milk  through  slovenly  arid  care- 
less methods  of  handling.  Those  species  associated  with 
ani  mal  excreta  are  particularly  dangerous.  The  number  of 
different  kinds  that  have  been  found  in  milk  is  quite  con- 
siderable, something  over  200  species  having  been  described 
more  or  less  thoroughly.  In  all  probability,  however,  many 
of  these  forms  will  be  found  to  be  identical  when  they  are 
subjected  to  a  more  critical  study. 

Direct  absorption  Of  taints.  A  tainted  condition  in  milk 
may  result  from  the  development  of  bacteria,  acting  upon 
various  constituents  of  the  milk,  and  transforming  these  in 
such  away  as  to  produce  by-products  that  impair  the  flavor 
or  appearance  of  the  liquid;  or  it  may  be  produced  by  the 
milk  being  brought  in  contact  with  any  odoriferous  or 
aromatic  substance,  under  conditions  that  permit  of  the 
direct  absorption  of  such  odors. 

This  latter  class  of  taints  is  entirely  independent  of  bac- 
terial action,  and  is  largely  attributable  to  the  physical 
property  which  milk  possesses  of  being  able  to  absorb  vola- 
tile odors,  the  fat  in  particular,  having  a  great  affinity  for 
many  of  these  substances.  This  direct  absorption  may 
occur  before  the  milk  is  withdrawn  from  the  animal,  or 
afterwards  if  exposed  to  strong  odors. 

It  is  not  uncommon  for  the  milk  of  animals  advanced  in 
lactation  to  have  a  more  or  less  strongly  marked  odor  and 
taste;  sometimes  this  is  apt  to  be  bitter,  at  other  times 
salty  to  the  taste.  It  is  a  defect  that  is  peculiar  to  indi- 
vidual animals  and  is  liable  to  recur  at  approximately  the 
same  period  in  lactation. 


56  Dairy  Bacteriology. 

The  peculiar  "cowy  "  or  "animal  odor"  of  fresh  milk  is 
an  inherent  peculiarity  that  is  due  to  the  direct  absorption 
of  volatile  elements  from  the  animal  herself.  This  condi- 
tion is  very  much  exaggerated  when  the  animal  consumes 
strong-flavored  substances  as  garlic,  leeks,  turnips  and  cab- 
bage. The  volatile  substances  that  give  to  these  vegetables 
their  characteristic  odor  are  quickly  diffused  through  the 
system,  and  if  such  foods  are  consumed  some  few  hours  be- 
fore milking,  the  odor  in  the  milk  will  be  most  pronounced. 
The  intensity  of  such  taints  is  diminished  greatly  and 
often  wholly  disappears,  if  the  milking  is  not  done  for  some 
hours  (8-12)  after  such  foods  are  consumed. 

This  same  principle  applies  in  lesser  degree  to  mary 
green  fodders  that  are  more  suitable  as  feed  for  animals,  as 
silage,  green  rye,  rape,  etc.  Not  infrequently,  such  fodders 
as  these  produce  so  strong  a  taint  in  milk  as  to  render  it 
useless  for  human  use.  Troubles  from  such  sources  could 
be  entirely  obviated  by  feeding  limited  quantities  of  such 
material  immediately  after  milking.  Under  such  condi- 
tions the  taint  produced  is  usually  eliminated  before  the 
next  milking.  The  milk  of  swill-fed  cows  is  said  to  possess 
a  peculiar  taste,  and  the  urine  of  animals  fed  on  this  food 
is  said  to  be  abnormally  acid.  Brewers'  grains  and  distil- 
lery slops  when  fed  in  excess  also  induce  a  similar  condi- 
tion in  the  milk. 

Milk  may  also  acquire  other  than  volatile  substances  di- 
rectly from  the  animal,  as  in  cases  where  drugs,  as  bella- 
donna, castor  oil,  sulfur,  turpentine,  jalap,  croton  oil, 
and  many  others  have  been  used  as  medicine.  Such  min- 
eral poisons  as  arsenic  have  been  known  to  appear  eight 
hours  after  ingestion,  and  persist  for  a  period  of  three 
weeks  before  being  eliminated. 


Contamination  of  Milk,  57 

Absorption  of  odors  after  milking:.  If  milk  is  brought  in 
contact  with  strong  odors  after  being  drawn  from  the  ani- 
mal, it  will  absorb  them  readily,  as  in  the  barn,  where  fre- 
quently it  is  exposed  to  the  odor  of  manure  and  other 
fermenting  organic  matter. 

It  has  long  been  a  popular  belief  that  milk  evolves 
odors  and  cannot  absorb  them  so  long  as  it  is  warmer  than 
the  surrounding  air,  but  from  experimental  evidence,  the 
writer1  has  definitely  shown  that  the  direct  absorption  of 
odors  takes  place  much  more  rapidly  when  the  milk  is 
warm  than  when  cold,  although  under  either  condition, 
it  absorbs  volatile  substances  with  considerable  avidity. 
In  this  test  fresh  milk  was  exposed  to  an  atmosphere  im- 
pregnated with  odors  of  various  essential  oils  and  other 
odor-bearing  substances.  Under  these  conditions,  the  cooler 
milk  was  tainted  very  much  less  than  the  milk  at  body 
temperature  even  where  the  period  of  exposure  was  brief. 
It  is  therefore  evident  that  an  exposure  in  the  cow  barn 
where  the  volatile  emanations  from  the  animals  themselves 
and  their  excreta  taint  the  air  will  often  result  in  the  ab- 
sorption of  these  odors  by  the  milk  to  such  an  extent  as 
to  seriously  affect  the  flavor. 

The  custom  of  straining  the  milk  in  the  barn  has  long 
been  deprecated  as  inconsistent  with  proper  dairy  prac- 
tice, and  in  the  light  of  the  above  experiments,  an  additional 
reason  is  evident  why  this  should  not  be  done. 

Even  after  milk  is  thoroughly  cooled,  it  may  absorb 
odors  as  seen  where  the  same  is  stored  in  a  refrigerator 
with  certain  fruits,  meats,  fish,  etc. 

Distinguishing  bacterial  from  non-bacterial  taints.    In 

perfectly  fresh  milk,  it  is  relatively  easy  to  distinguish  be- 

*  Russell,  15  Kept.  Wis.  Expt.  Stat.  1898,  p.  104. 


58  Dairy  Bacteriology. 

tween  taints  caused  by  the  growth  of  bacteria  and  those 
attributable  to  direct  absorption. 

If  the  taint  is  evident  at  time  of  milking,  it  is  in  all 
probability  due  to  character  of  feed  consumed,  or  possibly 
to  medicines.  If,  however,  the  intensity  of  the  taint  grows 
more  pronounced  as  the  milk  becomes  older,  then  it  is 
probably  due  to  living  organisms,  which  require  a  certain 
period  of  incubation  before  their  fermentative  properties 
are  most  evident. 

Moreover,  if  the  difficulty  is  of  bacterial  origin,  it  can  be 
frequently  transferred  to  another  lot  of  milk  (heated  or 
sterilized  is  preferable)  by  inoculating  same  with  some  of 
the  original  milk.  Not  all  abnormal  fermentations  are 
able  though  to  compete  with  the  lactic  acid  bacteria,  and 
hence  outbreaks  of  this  sort  soon  die  out  by  the  re-estab- 
lishment of  more  normal  conditions. 

Treatment  of  directly  absorbed  taints.  Much  can  be 
done  to  overcome  taints  of  this  nature  by  exercising  greater 
care  in  regard  to  the  feed  of  animals,  and  especially  as  to 
the  time  of  feeding  and  milking.  But  with  milk  already 
tainted,  it  is  often  possible  to  materially  improve  its  con- 
dition. Thorough  aeration  has  been  frequently  recom- 
mended, but  most  satisfactory  results  have  been  obtained 
where  a  combined  process  of  aeration  and  pasteurization 
was  resorted  to.  Where  the  milk  is  used  in  making  but- 
ter,, the  difficulty  has  been  successfully  met  by  washing  the 
cream  with  twice  its  volume  of  hot  water  in  which  a  little 
saltpeter  has  been  dissolved  (one  teaspoonful  per  gallon), 
and  then  separating  it  again.1 

The  treatment  of  abnormal  conditions  due  to  bacteria 
has  been  given  already  under  the  respective  sources  of  in- 

i  Alvord,  Circ.  No.  9,  U.  S.  Dept.  Agric.  (Div.  of  Sot.). 


Contamination  of  Milk.  59 

fection,  and  is  also  still  further  amplified  in  following 
chapter. 

Aeration.  Practical  experience  has  long  demonstrated 
the  advantage  of  aerating  the  milk  as  soon  after  milking 
as  possible.  This  is  accomplished  in  a  variety  of  ways.  In 
some  cases,  air  is  forced  into  the  milk;  in  others,  the  milk 
is  allowed  to  distribute  itself  in  a  thin  sheet  over  a  broad 
surface  and  fall  some  distance  so  that  it  is  brought  inti- 
mately in  contact  with  the  air.  The  benefit  claimed  for 
aeration  is  that  foul  odors  and  gases  which  may  be  present 
in  the  milk  are  thus  allowed  to  escape  by  bringing  the 
finely  divided  milk  into  contact  with  the  air.  As  ordi- 
narily practiced,  aeration  is  usually  combined  with  cooling, 
and  it  is  noteworthy  that  the  most  effective  aerators  are 
those  that  cool  simultaneously.  Under  these  conditions, 
the  keeping  quality  of  the  milk  is  increased,  but  where 
milk  is  simply  aerated  without  cooling,  no  material  benefit 
in  keeping  quality  is  observed.  A  satisfactory  scientific 
explanation  of  the  advantages  of  aeration  has  not  yet  been 
made.  It  is  difficult  to  see  how  the  process  can  have  any 
effect  on  the  bacterial  life  in  the  milk.  Its  influence,  un- 
doubtedly, is  on  the  odors  directly  absorbed  by  the  milk. 

Infection  of  milk  in  the  factory.  The  problem  of  proper 
handling  of  milk  is  not  entirely  solved  when  the  milk  is 
delivered  to  the  factory  or  creamery,  although  it  might  be 
said  that  the  danger  of  infection  is  much  greater  Avhile  the 
milk  is  on  the" form.  Then,  too,  contamination  of  milk  at 
time  of  withdrawal  gives  a  longer  period  of  incubation  for 
the  bacteria  in  the  milk,  and  consequently  intensifies  the 
effect  which  they  produce. 

In  the  factory,  infection  can  be  minimized  because  ef- 
fective measures  of  cleanliness  can  be  more  easity  applied. 


60  Dairy  Bacteriology. 

Steam  is  available  in  most  cases,  so  that  all  vats,  cans, 
churns  and  pails  can  be  thoroughly  scalded.  Special  em- 
phasis should  be  given  to  the  matter  of  cleaning  pumps 
and  pipes.  The  difficulty  of  keeping  these  utensils  clean 
often  leads  to  neglect  and  subsequent  infection.  Care 
must  be  taken  relative  to  the  use  of  worn  apparatus.  All 
cans  with  rusty  seams  should  be  discarded.  Permit  no 
vat  to  be  repaired  by  putting  in  a  false  covering  over  the 
old  one.  If  a  minute  leak  is  established,  such  places  be- 
come a  harbor  of  refuge  for  all  kinds  of  putrefactive  or- 
ganisms. In  a  number  of  cases  ill-smelling  factory  odors 
have  been  traced  to  such  a  cause. 

The  influence  of  the  air  on  the  germ  content  of  the  milk 
is,  as  a  rule,  overestimated.  If  the  air  is  quiet,  and  free 
from  dust,  the  amount  of  germ  life  in  the  same  is  not  rela- 
tively large.  In  a  creamery  or  factory,  infection  from  this 
source  ought  to  be  much  reduced,  for  the  reason  that  the 
floors  and  wall  are,  as  a  rule,  quite  damp,  and  hence  germ 
life  cannot  easily  be  dislodged.  The  majority  of  organisms 
found  under  such  conditions  come  from  the  person  of  the 
operators  and  attendants.  Any  infection  can  easily  be 
prevented  by  having  the  ripening  cream-vats  covered  with 
a  canvas  cloth.  The  clothing  of  the  operator  should  be 
different  from  the  ordinary  wearing-apparel.  If  made  of 
white  duck,  the  presence  of  dirt  is  more  quickly  recog- 
nized, and  greater  care  will  therefore  be  taken  than  if  or- 
dinary clothes  are  worn. 

The  surroundings  of  the  factory  have  much  to  do  with 
the  danger  of  germ  infection.  Many  factories  are  poorly 
constructed  and  the  drainage  is  poor,  so  that  filth  and  slime 
collect  about  and  especially  under  the  factory.  The  ema- 
nations from  these  give  the  peculiar  "  factory  odor  "  that 
indicates  fermenting  matter.  Not  only  are  these  odors 


Contamination  of  Milk.  61 

absorbed  directly,  but  germ  life  from  the  same  is  apt  to 
find  its  way  into  the  milk.  Connell1  has  recently  re- 
ported a  serious  defect  in  cheese  that  was  traced  to  germ 
infection  from  defective  factory  drains. 

The  water  supply  of  a  factory  is  also  a  question  of  prime 
importance.  When  taken  from  a  shallow  well,  especially 
if  surface  drainage  from  the  factory  is  possible,  the  water 
may  be  contaminated  to  such  an  extent  as  to  introduce 
undesirable  bacteria  in  such  numbers  that  the  normal 
course  of  fermentation  may  be  changed.  The  quality  of 
the  water,  aside  from  flavor,  can  be  best  determined  by 
making  a  curd  test  (p.  76)  which  is  done  by  adding  some 
of  the  water  to  boiled  milk  and  incubating  the  same.  If 
"  gassy  "  fermentations  occur,  it  signifies  an  abnormal  con- 
dition. In  deep  wells,  pumped  as  thoroughly  as  is  gener- 
erally  the  case  with  factory  wells,  the  germ  content  should 
be  very  low,  ranging  from  a  few  score  to  a  few  hundred 
bacteria  per  cc.  at  most. 

Harrison 2  has  recently  traced  an  off-flavor  in  cheese  in 
a  Canadian  factory  to  an  infection  arising  from  the  water- 
supply.  He  found  the  same  germ  in  both  water  and  cheese 
and  by  inoculating  a  culture  into  pasteurized  milk  suc- 
ceeded in  producing  the  undesirable  flavor.  The  danger 
from  ice  is  much  less,  for  the  reason  that  good  dairy  prac- 
tice does  not  sanction  using  ice  directly  in  contact  with 
milk  or  cream.  Then,  too,  ice  is  largely  purified  in  the 
process  of  freezing,  although  if  secured  from  a  polluted 
source,  reliance  should  not  be  placed  in  the  method  of 
purification;  for  even  freezing  does  not  destroy  all  vege- 
tating bacteria. 

i  Connell,  Kept,  of  Commissioner  of  Agr .,  Canada,  1897,  part  XVI,  p.  15. 
9  Harrison,  Hoard's  Dairyman,  March  4, 1898. 


CHAPTER  IV. 

FERMENTATIONS  IN  MILK  AND  THEIR 
TREATMENT. 

IT  has  been  shown  in  the  preceding  chapter  that  the 
contamination  of  milk  with  bacteria  occurs  so  constantly 
that  under  normal  conditions  it  always  contains  a  varying 
amount  of  germ  life.  The  result  of  this  infection  is  to 
cause  in  the  milk,  in  due  course  of  time,  subsequent  changes 
of  a  fermentative  nature.  As  a  rule  milk  sours,  due  to  the 
production  of  acid  from  the  decomposition  of  the  milk 
sugar;  but,  not  infrequently,  this  more  common  change  is 
supplanted  by  other  types  of  fermentative  activity  that  re- 
sult in  the  production  of  other  kinds  of  by-products.  These 
fermentations  are  sometimes  designated  as  abnormal,  be- 
cause of  their  less  frequent  occurrence. 

It  is  impossible  in  the  present  state  of  knowledge  to  sat- 
isfactorily classify  these  changes;  but  provisionally,  they 
may  be  grouped  according  to  the  substances  on  which  they 
act,  or  on  the  basis  of  the  most  prominent  by-product 
formed. 

Milk  is  such  a  complex  substance  that  the  changes  pro- 
duced by  a  single  germ  are  often  so  numerous  that  the 
processes  cannot  be  separated  in  their  reactions.  It  must 
be  remembered  then,  in  referring  to  the  different  types  of 
fermentations,  that  perhaps  a  distinct  by-product  is  being 
formed,  but  it  is  more  than  probable  that  there  are  a  series 
of  changes,  in  which  the  most  marked  decomposition  by- 
product is  alone  taken  into  consideration.  For  example, 
there  is  a  fermentation  classed  under  the  head  of  the  bu- 


Fermentations  in  Milk.  63 

tyric  changes,  a  decomposition  process  in  which  butyric 
acid  is  the  chief  product  formed,  but  this  may  be  associ- 
ated with  an  alkaline  condition  of  the  milk  and  the  pro- 
duction of  a  bitter  substance  in  the  same.  Thus,  the  sub- 
division followed  here  will ,  of  necessity  be  imperfect,  and 
occasional  instances  will  be  noted  where  some  changes  in 
milk  might  well  be  described  under  several  heads.  It  is 
possible  that  milk  may  acquire  an  abnormal  odor  or  taint, 
such  as  is  due  to  direct  absorption,  without  having  under- 
gone any  fermentative  change,  but  the  introduction  of 
various  forms  of  bacteria  is  so  common  that  fermentative 
changes  due  to  living  ferments  are  constantly  at  work. 

Souring  Of  milk.  Milk  naturally  undergoes  a  change 
known  as  souring,  if  allowed  to  stand  for  several  days  at 
ordinary  temperature.  This  is  due  to  the  formation  of 
lactic  acid,  which  is  produced  by  the  decomposition  of  the 
milk-sugar.  While  this  change  is  wellnigh  universal,  it 
does  not  occur  without  a  pre-existing  cause,  and  that  is  the 
presence  of  certain  living  bacterial  forms.  These  organ- 
isms develop  in  milk  with  great  rapidity,  and  the  decom- 
position changes  that  are  noted  in  souring  are  due  to  the 
by-products  of  their  development. 

The  milk-sugar  undergoes  fermentation,  the  chief  pro- 
duct being  lactic  acid,  although  various  other  by-products, 
.as  other  organic  acids  (acetic,  formic  and  succinic),  dif- 
ferent alcohols  and  gaseous  products,  as  C02,  H,  N  and 
methane  (CH4)  are  produced  in  small  amounts. 

In  this  fermentation,  the  acidity  begins  to  be  evident  to 
the  taste  when  it  reaches  about  0.3  per  cent,  calculated  as 
lactic  acid.  As  the  formation  of  acid  goes  on,  the  casein 
is  precipitated  and  incipient  curdling  or  lobbering  of  the 
milk  occurs.  This  begins  to  be  apparent  when  the  acidity 


64  Dairy  Bacteriology. 

is  about  0.4  per  cent,  but  the  curd  becomes  more  solid 
with  increasing  acidity.  The  action  of  the  bacteria  is  con- 
tinued until  about  0.8  to  1.0  per  cent  acid  is  formed,  although 
the  maximum  amount  fluctuates  considerably  with  differ- 
ent organisms.1  Further  formation  then  ceases,  by  reason 
of  the  inability  of  the  lactic  acid  organisms  to  continue 
their  development  in  such  acid  solutions.  There  is  always 
left  in  the  milk  a  considerable  amount  of  unfermented 
milk-sugar  which  can  be  further  acted  upon  by  the  con- 
tinued growth  of  the  bacteria,  if  a  carbonate  is  added  to 
the  milk  to  neutralize  the  developing  acid  that  inhibits 
their  growth. 

Cream  never  develops  as  much  acid  as  milk,  because  a 
larger  proportion  of  its  volume  is  made  up  of  butter-fat 
which  is  not  subject  to  this  change.  In  the  ripening  of 
cream  in  butter-making,  it  is  necessary  to  take  this  fact 
into  consideration  where  the  cream  varies  widely  in  per 
cent  of  fat. 

The  formation  of  lactic  acid  is  a  characteristic  that  is 
possessed  by  a  large  number  of  bacteria,  micrococci  as  well 
as  bacilli  being  numerously  represented.  Still  the  pre- 
ponderance of  evidence  is  in  favor  of  the  view  that  one 
main  type  is  responsible  for  most  of  this  fermentation. 
The  most  prominent  organism  associated  with  this  change 
is  Bacillus  acidi  lactici,  first  described  by  Hiippe.9  Giin- 
therand  Thierfelder3  working  on  the  spontaneous  souring 
of  milk  in  the  neighborhood  of  Berlin  found  what  they 
think  is  the  same  germ.  Esten,4  in  this  country,  studied 
milks  from  thirty  different  localities  in  New  England  and 

>  Warrington,  Jour.  Chem.  Soc.,  1888,  53:727. 
8  HUppe,  Mitt.  a.  cL  k.  Gesundheitsamte,  1884,  2:309. 
*  Gtinther  and  Thierfelder,  Arch.  f.  Hyg.,  25:164. 
«  Esten,  9  Kept.  Storrs  Expt  Stat.,  1896,  p.  44. 


Fermentations  in  Milk.  65 

the  Middle  States.  He  found  a  germ  in  all  but  two  cases 
that  agreed  in  general  with  Gunther's  description.  Din- 
widdie,1  studying  the  same  question  in  Arkansas,  arrives 
at  the  same  conclusion.  This  preponderance  of  evidence 
makes  it  quite  probable  that  there  is  a  widely  distributed 
germ  that  is  concerned  in  this  change  although  undoubt- 
edly different  varieties  exist.  Besides  this  widely  dissemi- 
nated type,  there  are  numerous  other  forms 2  that  are 
associated  with  this  type  of  decomposition  the  most  promi- 
nent of  which  is  known  as  B.  lactis  aerogenes.  Conn  and 
Aikman  refer  to  the  fact  that  over  one  hundred  species 
are  already  known.  It  is  fair  to  presume,  however,  that 
a  careful  comparative  study  of  these  would  show  that 
simply  racial  differences  exist  in  many  cases,  and  therefore, 
that  they  are  not  distinct  species. 

This  class  of  bacteria  is  characterized  by  their  inability 
to  liquefy  gelatin  or  develop  spores.  On  account  of  this 
latter  characteristic  they  are  easily  destroyed  when  milk  is 
pasteurized.  They  live  under  aerobic  or  anaerobic  condi- 
tions, many  of  them  being  able  to  grow  in  either  environ- 
ment, although,  according  to  McDonnell,3  they  are  more 
virulent  when  air  is  not  excluded. 

The- temperature  conditions  as  to  growth  vary  somewhat 
with  different  species.  With  most  species  this  occurs  at 
50°  F.,  but  appreciable  amounts  of  acid  are  not  produced 
until  a  higher  temperature  is  reached.4  The  optimum 
temperatures  for  growth  range  from  90°-95°  F. 

While  the  souring  of  milk  is  a  very  wide-spread  phenom- 
enon, still  lactic  acid  organisms  are  not  universal^  dis- 

1  Bull.  45,  Ark.  Expt.  Stat.,  May  1897;  Leichmann,  Hyg.  Rund.,  1899,  p.  1267. 

2  Kayser,  Ann.  de  Tlnst  Past.,  10:737. 

3  McDonnell,  Inaug.  Diss.,  Kiel,  1899,  p.  39. 
*  Kayser,  Cent.  f.  Bakt.,  II  Abt.  1:436. 

5 


66  Dairy  Bacteriology. 

tributed.  According  to  Conn '  they  are  not  very  abun- 
dant in  perfectly  fresh  milk,  but  because  of  their  ability  to 
thrive  so  luxuriantly  in  this  liquid,  they  grow  with  great 
rapidity,  and  therefore  after  a  few  hours  milk  always  con- 
tains them  in  abundance. 

It  is  a  wide-spread  belief  that  thunder  storms  cause  milk 
to  sour  prematurely,  but  this  idea  has  no  scientific  founda- 
tion. Experiments9  with  the  electric  spark,  ozone  and 
loud  detonations  show  no  effect  on  acid  development,  but 
the  atmospheric  conditions  usually  incident  to  a  thunder 
storm  are  such  as  permit  of  a  more  rapid  growth  of  organ- 
isms. There  is  no  reason  to  believe  but  that  the  phenom- 
enon of  souring  is  wholly  related  to  the  development  of 
bacteria.  Sterile  milks  are  never  affected  by  the  action  of 
electric  storms. 

"  Gassy "  milks.  A  large  number  of  bacteria  possess 
the  property  of  fermenting  sugars  and  producing  gases  of 
various  kinds  as  well  as  acids  of  a  volatile  or  fixed  char- 
acter. The  amount  of  acid  formed  is  generally  consider- 
ably less  than  that  produced  by  the  normal  lactic  species. 
Among  the  gases  formed,  H  and  C02  are  most  common, 
although  N  and  CH4  (methane)  are  sometimes  produced. 
In  connection  with  these  gases,  there  are  also  other  de- 
composition products  of  a  more  or  less  volatile  nature  that 
frequently  impart  to  the  milk  taints  of  an  undesirable 
character. 

While  these  "  gassy  "  defects  can  often  be  recognized  in 
the  milk  itself,  they  are  much  more  apt  to  cause  trouble 
in  the  manufacture  of  cheese  (see  Fig.  16),  where,  in  severe 
cases,  curds  may  " float"  or  be  "pin  holey."*  There  are 

1  Conn,  Agricultural  Bacteriology,  p.  191. 
»  Treadwell,  Science,  1804, 17: 178. 

»  Freudenreich,  Landw.  Jahr.  cL  Schweiz,  1890,  p.  17;  Russell,  12  Eept  Wte. 
Ezpt.  Stat.,  1895,  p.  189. 


Fermentations  in  Milk.  67 

a  large  number  of  organisms  of  this  class  found  in  surface 
waters,  soils  and  in  decomposing  organic  matter.  The 
colon  bacillus  of  the  intestinal  tract  is  a  germ  of  this  type 


FIG.  16.    Cheese  made  from  "  gassy  "  milk. 

that  finds  its  way  into  milk  with  manure  particles.  B.  lactis 
aerogenes,  a  common  inhabitant  of  milk  is  also  a  gas-pro- 
ducer. Abnormal  fermentations  of  this  class  occur  most 
frequently  in  the  hot  summer  months,  but  are  not  neces- 
sarily confined  to  this  season.  Wherever  carelessness  pre- 
vails in  the  matter  of  cleaning  utensils,  troubles  from  gassy 
milks  are  very  apt  to  occur. 

"Sweet  curdling"  and  digesting:  fermentations.    Not 

infrequently  milk,  instead  of  undergoing  spontaneous  sour- 
ing, curdles  in  a  weakly  acid  or  neutral  condition,  in  which 
state  it  is  said  to  have  undergone  "  sweet  curdling."  The 
coagulation  of  the  milk  is  caused  by  the  action  of  enzyms 
of  a  rennet  type  that  are  formed  by  the  growth  of  various 
species  of  bacteria.  Later  the  whey  separates  more  or  less 
perfectly  from  the  curd,  producing  a  "wheyed  off"  condi- 
tion. Generally  the  coagulum  in  these  cases  is  soft  and 


68  Dairy  Bacteriology. 

somewhat  slimy.  The  curd  usually  diminishes  in  bulk, 
due  to  the  gradual  digestion  or  peptonization  of  the  casein 
by  proteid-dissolving  enzyms  (tryptic  type)  that  are  also 
produced  by  the  bacteria  causing  the  change. 

A  large  number  of  bacteria  possess  the  property  of  af- 
fecting milk  in  the  above  way.  Generally  they  are  able 
to  liquefy  gelatin  (also  a  peptonizing  process)  and  form 
spores.  The  Tyrothrix  type  of  bacteria  (so  named  by  Du- 
claux  on  account  of  the  supposed  relation  to  cheese  ripen- 
ing) belongs  to  this  class.  The  hay  and  potato  forms  are 
also  digesters.  Organisms  of  this  type  are  generally  as- 
sociated with  filth  and  manure,  and  find  their  way  into 
the  milk  from  the  accumulations  on  the  coat  of  the  aniinal. 

Conn '  has  separated  the  rennet  enzym  from  bacterial 
cultures  in  a  relatively  pure  condition,  while  Fermi'2  has 
isolated  the  digestive  principle  from  several  species. 

Duclaux3  has  given  to  this  digesting  enzym  the  name 
casease  or  cheese  ferment.  These  isolated  ferments  when 
added  to  fresh  milk  possess  the  power  of  causing  the  char- 
acteristic curdling  and  subsequent  digestion  quite  inde- 
pendent of  cell  development.  The  quantity  of  ferment 
produced  by  different  species  differs  materially  in  some 
cases.  In  these  digestive  fermentations,  the  chemical  trans- 
formations are  profound,  the  complex  proteid  molecule 
being  broken  down  into  albumoses,  peptones,  amido-acids 
(ty rosin  and  leucin)  and  ammonia  as  well  as  fatty  acids. 

Not  infrequently  these  fermentations  gain  the  ascend- 
ency over  the  normal  souring  change,  but  under  ordinary 
conditions  they  are  held  in  abeyance,  although  this  type  of 
bacteria  is  always  present  to  some  extent  in  milk.  When 

'  Conn,  5  Kept,  Storrs  Expt.  Stat..  1892.  p.  396. 
»  Fermi,  Arch.  f.  Hyg.,  1892, 14:1. 
*  Duclaux,  Le  Lait,  p.  121. 


Fermentations  in  Milk.  69 

the  lactic  acid  bacteria  are  destroyed,  as  in  boiled,  sterilized 
or  pasteurized  milk,  these  rennet-producing,  digesting 
species  develop. 

Butyric  acid  fermentations.  The  formation  of  butyric 
acid  in  milk  which  may  be  recognized  by  the  "rancid  but- 
ter" odor  is  not  infrequently  seen  in  old,  sour  milk,  and 
for  a  long  time  was  thought  to  be  a  continuation  of  the 
lactic  fermentation,  but  it  is  now  believed  that  these  or- 
ganisms find  more  favorable  conditions  for  growth,  not  so 
much  on  account  of  the  lactic  acid  formed  as  in  the  ab- 
sence of  dissolved  oxygen  in  the  milk  which  is  consumed 
by  the  sour-milk  organisms. 

Most  of  the  butyric  class  of  bacteria  are  spore-bearing, 
and  hence  they  are  frequently  present  in  boiled  or  steril- 
ized milk.  The  by-products  formed  in  this  series  of  changes 
are  quite  numerous.  In  most  cases,  butyric  acid  is  promi- 
nent, but  in  addition  to  this,  other  organic  acids,  as  lactic, 
succinic,  and  acetic,  are  produced,  likewise  different  alco- 
hols. Concerning  the  chemical  origin  of  butyric  acid 
there  is  yet  some  doubt.  Duclaux l  affirms  that  the  fat, 
sugar  and  casein  are  all  decomposed  by  various  forms. 
In  some  cases,  the  reaction  of  the  milk  is  alkaline,  with 
other  species  it  may  be  neutral  or  acid.  This  type  of  fer- 
mentation has  not  received  th'e  study  it  deserves. 

In  milk  these  organisms  are  not  of  great  importance,  as 
this  fermentation  does  not  readily  gain  the  ascendency 
over  the  lactic  bacteria. 

Ropy  or  slimy  milk.  The  viscosity  of  milk  is  often 
markedly  increased  over  that  which  it  normally  possesses. 
The  intensity  of  this  abnormal  condition  may  vary  much; 
in  some  cases  the  milk  becoming  viscous  or  slimy;  in  others 

1  Duclaux,  Principes  de  Laiterie,  p.  67. 


70 


Dairy  Bacteriology. 


stringing  out  into  long  threads,  several  feet  in  length,  as 
in  Fig.  17.  Two  sets  of  conditions  are  responsible  for  these 
ropy  or  slimy  milks.  The  most  com- 
mon is  where  the  milk  is  clotted  or 
stringy  when  drawn,  as  in  some  forms 
of  garget.  This  is  generally  due  to  the 
presence  of  viscid  pus,  and  is  often  ac- 
companied by  a  bloody  discharge,  such 
a  condition  representing  an  inflamed 
state  of  the  udder.  Ropiness  of  this 
character  is  not  usually  communicable 
from  one  lot  of  milk  to  another. 

The  communicable  form  of  ropy  milk 
only  appears  after  the  milk  has  been 
drawn  from  the  udder  for  a  day  or  so, 
and  is  caused  by  the  development  of 
various  species  of  bacteria  which  find 
their  way  into  the  milk  after  it  is  drawn. 
These  defects  are  liable  to  occur  at  any 
season  of  the  year.  Their  presence  in 
a  dairy  is  a  source  of  much  trouble,  as 
the  unsightly  appearance  of  the  milk 
precludes  its  use  as  food,  although  there 
is  no  evidence  that  these  ropy  fermen-  Fl°- 17-  R°PV  milk- 
tations  are  dangerous  to  health. 

There  are  undoubtedly  a  number  of  different  species  of 
bacteria  that  are  capable  of  producing  these  viscid  changes,1 
but  it  is  quite  probable  that  they  are  not  of  equal  im- 
portance in  infecting  milk  under  natural  conditions. 

In  the  majority  of  cases  studied  in  this  country,*  the 

1  Guillebeau  (Milch  Zeit.,  1892,  p.  808)  has  studied  over  a  dozen  different  forms 
that  possess  this  property. 
*  Ward,  Bull.  165,  Cornell  Expt.  Stat.,  Mch.,  1899;  also  Bull.  195,  Ibid.,  Nov.,  1901. 


Fermentations  in  Milk.  71 

causal  organism  seems  to  be  B.  lactis  viscosus.  a  form  first 
found  by  Adametz  in  surface  waters.1  This  organism  pos- 
sesses the  property  of  developing  at  low  temperatures 
(45°-50°  F.),  and  consequently  it  is  often  able  in  winter 
to  supplant  the  lactic-acid  forms.  Ward  has  found  this 
germ  repeatedly  in  water  tanks  where  milk  cans  are  cooled; 
and  under  these  conditions  it  is  easy  to  see  how  infection 
of  the  milk  might  occur.  Marshall 2  reports  an  outbreak 
which  he  traced  to  an  external  infection  of  the  udder;  in 
another  case,  the  slime-forming  organism  was  abundant 
in  the  barn  dust.  A  defect  of  this  character  is  often  per- 
petuated in  a  dairy  for  some  time,  and  may  therefore  be- 
come exceedingly  troublesome.  In  one  instance  in  the 
writer's  experience,  a  milk  dealer  lost  over  $150  a  month 
for  several  months  from  ropy  cream.  Failure  to  properly 
sterilize  cans,  and  particularly  strainer  cloths,  is  frequently 
responsible  for  a  continuance  of  trouble  of  this  sort. 

The  slimy  substance  formed  in  milk  comes  from  vari- 
ous constituents  of  the  milk,  and  the  chemical  character 
of  the  slime  produced  also  varies  with  different  germs.  In 
some  cases  the  slimy  material  is  merely  the  swollen  outer 
cell  membrane  of  the  bacteria  themselves  as  in  the  case  of 
B.  lactis  viscosus;  in  others  it  is  due  to  the  decomposition 
of  the  proteids,  but  often  the  chief  decomposition  product 
appears  to  come  from  a  viscous  fermentation  of  the  milk- 
sugar. 

An  interesting  case  of  a  fermentation  of  this  class  being 
utilized  in  dairying  is  seen  in  the  use  of  ulangewei" 
(long  or  stringy  whey)  which  is  employed  as  a  starter  in 
Holland  to  control  the  gassy  fermentations  in  Edam  cheese. 

1  Adametz,  Landw.  Jahr.,  1891,  p.  185. 

2  Marshall,  Mich.  Expt.  Stat.,  Bull.  140. 


72  Dairy  Bacteriology. 

This  slimy  change  is  due  to  the  growth  of  Streptococcus 
Hollandicus.1 

Alcoholic  fermentations.  Although  glucose  or  cane- 
sugar  solutions  are  extremely  prone  to  undergo  alcoholic 
fermentation,  milk  sugar  does  not  readily  decompose.  The 
more  important  alcoholic  ferments  are  the  yeasts,  which 
do  not  thrive  readily  in  the  milk,  although  Duclaux2  re- 
ports a  serious  case  in  a  dairy  due  to  this  cause. 

Koumiss,  a  beverage  originally  made  in  the  Orient  from 
mare's  milk,  is  an  example  of  an  alcoholic  fermentation 
which  is  produced  hy  the  addition  of  cane  sugar  and  yeast 
to  ordinary  cow's  milk.  It  is  used  with  success  in  gastric 
troubles.  In  addition  to  the  C02  developed  which  gives  it 
its  effervescent  qualities,  alcohol,  lactic  acid,  and  casein- 
dissolving  ferments  are  also  formed. 

Kephir  is  another  alcoholic  drink  made  from  milk  that 
is  in  common  use  among  the  people  of  Caucasus.  It  is 
made  by  adding  to  milk  kephir  grains,  which  are  merely  a 
mass  of  fermented  cells  (yeasts  and  bacteria)  that  start  the 
fermentation.  This  milk  is  then  mixed  with  fresh  milk 
and  kept  in  leather  flasks  until  a  mixed  fermentation  sets 
in.  The  nature  of  the  change  is  not  yet  thoroughly  under- 
stood,8 although  it  is  quite  probable  that  the  alcoholic 
change  is  produced  by  a  yeast,  while  bacteria  change  the 
casein  more  or  less. 

Bitter  milk.  The  presence  of  bitter  substances  in  milk 
may  be  ascribed  to  a  variety  of  causes.  A  number  of  plants, 
such  as  lupines,  wormwood  and  chicory,  possess  the  prop- 
erty of  affecting  milk  when  the  same  are  consumed  by  ani- 

'  Milch  Zeit.,  1889,  p.  982. 

'Duclaux,  Principes  de  Laiterie.  p.  60. 

'  Freudenreich,  Landw.  Jahr.  d.  Schweiz,  1896, 10:.l. 


Fermentations  in  Milk.  73 

mals.  At  certain  stages  in  lactation,  a  bitter  salty  taste  is 
occasionally  to  be  noted  that  is  peculiar  to  individual  ani- 
mals. 

A  considerable  number  of  cases  of  bitter  milk  have,  how- 
ever, been  traced  to  bacterial  origin.  For  a  number  of 
years  the  bitter  fermentation  of  milk  was  thought  to  be 
associated  with  the  butyric  fermentation,  but  Weigmann1 
showed  that  the  two  conditions  were  not  dependent  upon 
each  other.  He  found  that  the  organism  which  produced 
the  bitter  taste  acted  upon  the  casein. 

Conn2  observed  a  coccus  form  in  bitter  cream  that  was 
able  to  impart  a  bitter  flavor  to  milk.  Sometimes  a  bitter 
condition  does  not  develop  in  the  milk,  but  may  appear 
later  in  the  milk  products,  as  in  the  case  of  a  micrococcus 
which  Freudenreich 3  found  in  cheese. 

Cream  ripened  at  low  temperatures  not  infrequently  de- 
velops a  bitter  flavor,  showing  that  the  optimum  tempera- 
ture for  this  type  of  fermentation  is  below  the  typical 
lactic  acid  change. 

It  has  long  been  a  question  how  to  account  chemically 
for  the  bitter  taste  in  milk.  Various  ideas  have  been  ad- 
vanced, but  Freudenreich  has  demonstrated  in  one  case 
that  a  bitter  substance  is  formed  in  the  milk  that  can  be 
isolated  by  adding  alcohol. 

Milk  that  has  been  cooked  is  likely  to  develop  a  bitter 
condition.  The  explanation  of  this  is  that  the  bacteria 
producing  the  bitter  substances  usually  possess  endospores, 
and  that  while  the  boiling  or  sterilizing  of  milk  easily 
kills  the  lactic  acid  germs,  these  forms  on  account  of  their 
greater  resisting  powers  are  not  destroyed  by  the  heat. 

i  Weigmann.  Milch  Zeit.,  1890,  p.  881. 

8  Conn,  3  Kept.  Storrs  Expt.  Stat.,  1890,  p.  153. 

a  Freudenreich,  Fiihl.  Landw.  Ztg.,  43:  361. 


74  Dairy  Bacteriology. 

Soapy  milk:  A  soapy  flavor  in  milk  was  traced  by  Weig- 
mann  and  Ziru  *  to  a  specific  bacillus,  B.  lactis  saponacei, 
that  they  found  gained  access  to  the  milk  in  one  case  from 
the  bedding  and  in  another  instance  from  ha}r.  A  similar 
outbreak  has  been  reported  in  this  country,2  due  to  a  germ 
acting  on  the  casein  and  albumen. 

Red  milk.  The  most  common  trouble  of  this  nature 
in  milk  is  due  to  presence  of  blood,  which  is  most  fre- 
quently caused  by  some  wound  in  the  udder.  The  inges- 
tion  of  certain  plants  as  sedges  and  scouring  rushes  is  also 
said  to  cause  a  bloody  condition;  madders  impart  a  red- 
dish tinge  due  to  coloring  matter  absorbed.  Defects  of  this 
class  can  be  readily  distinguished  from  those  due  to  germ 
growth  because  they  are  apparent  at  time  of  milking. 
Where  blood  is  actually  present,  the  corpuscles  settle  out 
in  a  short  time  if  left  undisturbed. 

There  are  a  number  of  chromogenic  or  color-producing 
bacteria  that  are  able  to  grow  in  milk,  but  their  action  is 
so  slow  that  generally  they  are  not  of  much  consequence. 
Moreover  their  development  is  usually  confined  to  the  sur- 
face of  the  milk  as  it  stands  in  a  vessel.  The  most  import- 
ant is  the  well-known  B.  prodigiosus.  Another  form  found 
at  times  in  milk  possessing  low  acidity3  is  B.  lactis  erythro- 
genes.  This  species  only  develops  the  red  color  in  the  dark. 
In  the  light,  it  forms  a  yellow  pigment.  Various  other 
organisms  have  been  reported  at  different  times.4 

Blue  milk.  Blue  milk  has  been  known  for  many  years, 
its  communicable  nature  being  established  as  long  ago  as 
1838.  It  appears  on  the  surface  of  milk  first  as  isolated 

»  Miloh  Zeit.  22:5(59. 

'Marshall,  Bull.  146,  Mich.  Expt.  Stat.,  p.  16. 

«  Grotenfelt,  Milch  Zeit.,  1833,  p.  283. 

«Menge,  Cent.  f.  Bakt.,  6:590;  Keferstein,  Cent.  f.  B?.kt.,  21:177. 


Fermentations  in  Milk.  75 

particles  of  bluish  or  grey  color,  which  later  become  con- 
fluent, the  blue  color  increasing  in  intensity  as  the  acidity 
increases.  The  causal  organism,  B.  cyanogenes,  is  very  re- 
sistant toward  drying,1  thus  accounting  for  its  persistence. 
In  Mecklenberg  an  outbreak  of  this  sort  once  continued 
for  several  years.  It  has  frequently  been  observed  in  Eu- 
rope in  the  past,  but  is  not  now  so  often  reported.  Occa- 
sional outbreaks  have  been  reported  in  this  country. 

Other  kinds  of  colored  milk.  Two  or  three  chromo- 
genic  forms  producing  still  other  colors  have  occasionally 
been  found  in  milk.  Adametz  9  discovered  in  a  sample  of 
cooked  milk  a  peculiar  form  (Bacillus  synxanthus)  that 
produced  a  citron-yellow  appearance  which  precipitated 
and  finally  rendered  soluble  the  casein.  Adanietz,  Conn, 
and  List  have  described  other  species  that  confer  tints  of 
yellow  on  milk.  Some  of  these  are  bright  lemon,  others 
orange,  and  some  amber  in  color. 

Still  other  color-producin  8  bacteria,  such  as  those  that 
produce  violet  or  green  changes  in  the  milk,  have  been  ob- 
served. In  fact,  almost  any  of  the  chromogenic  bacteria 
are  able  to  produce  their  color  changes  in  milk  as  it  is  such 
an  excellent  food  medium.  Under  ordinary  conditions, 
these  do  not  gain  access  to  milk  in  sufficient  numbers  so 
that  they  modify  the  appearance  ot  it  except  in  occa.jional 
instances. 

Treatment  of  abnormal  fermentations.  If  the  taint  is 
recognized  as  of  bacterial  origin  (see  p.  57)  and  is  found  in 
the  mixed  milk  of  the  herd,  it  is  necessary  to  ascertain, 
first,  whether  it  is  a  general  trouble,  or  restricted  to  one  or 
more  animals.  This  can  sometimes  be  done  by  separating 

J  Helm,  Arb.  a.  d.  Kais.  Gesundheitsamte,  5:578. 
a  Adametz,  Milch  Zeit.,  1890,  p.  225. 


76  Dairy  Bacteriology. 

the  milk  of  the  different  cows  and  noting  whether  any  ab- 
normal condition  develops  in  the  respective  samples. 

Fermentation  tests.  The  moot  satisfactory  way  to  de- 
tect the  presence  of  the  taints  more  often  present  is  to 
make  a  fermentation  test  of  one  kind  or  another.  These 
tests  are  most  frequently  used  at  the  factory,  to  enable  the 
maker  to  detect  the  presence  of  milk  that  is  likely  to  prove 
unfit  for  use,  especially  in  cheese  making.  They  are  based 
upon  the  principle  that  if  milk  is  held  at  a  moderately  high 
temperature,  the  bacteria  will  develop  rapidly.  A  number 
of  different  methods  have  been  devised  for  this  purpose.  In 
Walther^s  lacto-fermentator  samples  of  milk  are  simply 
allowed  to  stand  in  bottles  or  glass  jars  until  they  sour. 
They  are  examined  at  intervals  of  several  hours.  If  the 
curdled  milk  is  homogeneous  and  has  a  pure  acid  smell,  the 
milk  is  regarded  as  all  right.  If  it  floats  in  a  turbid  serum, 
is  full  of  gas  or  ragged  holes,  it  is  abnormal.  As  generally 
carried  out,  no  attempt  is  made  to  have  these  vessels  ster- 
ile. Gerber's  test  is  a  similar  test  that  has  been  extensively 
employed  in  Switzerland.  Sometimes  a  few  drops  of  rennet 
are  added  to  the  milk  so  as  to  curdle  the  same,  and  thus 
permit  of  the  more  ready  detection  of  the  gas  that  is  evolved. 

Wisconsin  curd  test.  The  method  of  testing  milk  de- 
scribed below  was  devised  at  the  Wisconsin  Experiment 
Station  in  1895  by  Babcock,  Russell  and  Decker.1  It  was 
used  first  in  connection  with  experimental  work  on  the 
influence  of  gas-generating  bacteria  in  cheese  making,  but 
its  applicability  to  the  detection  of  all  taints  in  milk  pro- 
duced by  bacteria  makes  it  a  valuable  test  for  abnormal 
fermentations  in  general. 

In  the  curd  test  a  small  pat  of  curd  is  made  in  a  glass 

» 12  Kept.  Wis.  Expt.  Stat.,  1895,  p.  148;  also  Bull.  67,  Ibid.,  June,  1898. 


Fermentations  in  Milk. 


77 


jar  from  each  sample  of  milk.  These  tests  may  be  made 
in  any  receptacle  that  has  been  cleaned  in  boiling  water, 
and  to  keep  the  temperature  more  nearly  uniform  these 
jars  should  be  immersed  in  warm  water,  as  in  a  wash  tub 
or  some  other  receptacle.  When  the  milk  is  about  95°  F., 
about  ten  drops  of  rennet  extract  are  added  to  each  sample 
and  mixed  thoroughly  with  the  milk.  The  jars  should 
then  remain  undisturbed  until  the  milk  is  completely 
curdled;  then  the  curd  is  cut  into  small  pieces  with  a  case 
knife  and  stirred  to  expel  the  whey.  The  whey  should  be 
poured  off  at  frequent  intervals  until  the  curd  mats.  If 
the  sample  be  kept  at  blood  heat  (98°  F.)  for  six  to  eight 
hours,  it  will  be  ready  to  examine. 


FIG.  18.  Improved  bottles  for  making  curd  test.  A,  test  bottle  complete;  B, 
bottle  showing  construction  of  cover;  S,  sieve  to  hold  back  the  curd  when  bottle 
is  inverted;  C,  outer  cover  with  (D  IT)  drain  holes  to  permit  of  removal  of  whey. 

More  convenient  types  of  this  test  than  the  improvised 
apparatus  just  alluded  to  have  been  devised  by  different 
dairy  manufacturers.  Generally,  they  consist  of  a  special 
bottle  having  a  full-sized  top,  thus  permitting  the  easy 


78  Dairy  Bacteriology. 

with  a  sieve  of  such  construction  that  the  bottles  will  drain 
thoroughly  if  inclined  in  an  inverted  position. 

Interpretation  of  results  of  test.  The  curd  from  a  good 
milk  has  a  firm,  solid  texture,  and  should  contain  at  most 
only  a  few  small  pin  holes.  It  may  have  some  large,  irreg- 
ular, ''mechanical1'  holes  where  the  curd  particles  have 
failed  to  cement,  as  is  seen  in  Fig.  19.  If  gas-producing  bac- 


Fto.  19.    Curd  from  a  good  milk.    The  large  Irregular  holes  are  Hiechanical. 

teria  are  very  prevalent  in  the  milk,  the  conditions  under 
which  the  test  is  made  cause  such  a  rapid  growth  of  the 
same  that  the  evidence  of  the  abnormal  fermentation  may 
bs  readily  seen  in  the  spongy  texture  of  the  curd  (Fig.  20). 
If  the  undesirable  organisms  are  not  very  abundant  and  the 
conditions  not  especially  suited  to  their  growth,  the  "  pin 
holes  "  will  be  less  frequent. 

Sometimes  the  curds  show  no  evidence  of  gas,  but  their 
abnormal  condition  can  be  recognized  by  the  "mushy" 
texture  and  the  presence  of  "  off"  flavors  that  are  rendered 
more  apparent  by  keeping  them  in  closed  bottles.  This 
condition  is  abnormal  and  is  apt  to  produce  quite  as  serious 
results  as  if  gas  was  formed. 


Fermentations  in  Milk.  79 

Overcoming  taints  by  use  of  starters.  Another  method 
of  combatting  abnormal  fermentations  that  is  often  fruit- 
ful, is  that  which  rests  upon  the  inability  of  one  kind  of 
bacteria  to  grow  in  the  same  medium  in  competition  with 
certain  other  species. 

Some  of  the  undesirable  taints  in  factories  can  be  con- 
trolled in  large  part  by  the  introduction  of  starters  made 


FIG.  20.    Curd  from  a  badly  tainted  milk.  Large  ragged  holes  are  mechanical; 
numerous  small  holes  due  to  gas.    This  curd  was  a  "  floater." 

from  certain  organisms  that  are  able  to  obtain  the  ascend- 
ency over  the  taint-producing  germ.  Such  a  method  is 
commonly  followed  when  a  lactic  ferment,  either  a  com- 
mercial pure  culture,  or  a  home-made  starter,  is  added  to 
milk  to  overcome  the  effect  of  gas-generating  bacteria. 

A  similar  illustration  is  seen  in  the  case  of  the  "  lange 
wei"  (slimy  whey),  that  is  used  in  the  manufacture  of 
Edam  cheese  to  control  the  character  of  the  fermentation 
of  the  milk. 

This  same  method  is  sometimes  applied  in  dealing  with 
certain  abnormal  fermentations  that  are  apt  to  occur  on 
the  farm.  It  is  particularly  useful  with  those  tainted  milks 
known  as  "  sweet  curdling."  The  ferment  organisms  con- 


80  Dairy  Bacteriology. 

cerned  in  this  change  are  unable  to  develop  in  the  presence 
of  lactic  acid  bacteria,  so  the  addition  of  a  clean  sour  milk 
as  a  starter  restores  the  normal  conditions  by  giving  the 
ordinary  milk  bacteria  the  ascendency. 

Chemical  disinfection.  In  exceptional  instances  it  may 
be  necessary  to  employ  chemical  disinfectants  to  restore 
the  normal  conditions.  Of  course  with  such  diseases  as 
tuberculosis,  very  stringent  measures  are  required,  as  they 
are  such  a  direct  menace  to  human  life,  but  with  these 
abnormal  or  taint-producing  fermentations,  care  and  clean- 
liness, well  directed,  will  usually  overcome  the  trouble. 

If  it  becomes  necessary  to  employ  chemical  substances 
as  disinfecting  agents,  their  use  should  always  be  preceded 
by  a  thorough  cleansing  with  hot  water  so  that  the  germi- 
cide may  come  in  direct  contact  with  the  surface  to  be 
disinfected. 

It  must  be  borne  in  mind  that  many  chemicals  act  as 
deodorants,  i.  e.,  destroy  the  offensive  odor,  without  destroy- 
ing the  cause  of  the  trouble. 

Sulfur  is  often  recommended  as  a  disinfecting  agent,  but 
its  use  should  be  carefully  controlled,  otherwise  the  vapors 
have  but  little  germicidal  power.  The  common  practice  of 
burning  a  small  quantity  in  a  room  or  any  closed  space  for 
a  few  moments  has  little  or  no  effect  upon  germ  life.  The 
effect  of  sulfur  vapor  (S02)  alone  upon  germ  life  is  relatively 
slight,  but  if  this  gas  is  produced  in  the  presence  of  mois- 
ture, sulfurous  acid  (H2S03)  is  formed,  which  is  much 
more  efficient.  To  use  this  agent  effectively,  it  must  be 
burned  in  large  quantities  in  a  moist  atmosphere  (three  Ibs. 
to  every  1,000  cubic  feet  of  space),  for  at  least  twelve  hours. 
After  this  operation,  the  space  should  be  thoroughly  aired. 

Formalin,  a  watery  solution  of  a  gas  known  as  form- 
aldehyde, is  a  new  disinfectant  that  recent  experience  has 


Fermentations  in  Milk.  81 

demonstrated  to  be  very  useful.  It  may  be  used  as  a  gas 
where  rooms  are  to  be  disinfected,  or  applied  as  a  liquid 
where  desired.  It  is  much  more  powerful  in  its  action  than 
sulfur,  and  it  has  a  great  advantage  over  mercury  and  other 
strong  disinfectants,  as  it  is  not  so  poisonous  to  man  as  it 
is  to  the  lower  forms  of  life. 

Bleaching  powder  or  chloride  of  lime  is  often  recommended 
where  a  chemical  can  be  advantageously  used.  This  sub- 
stance is  a  good  disinfectant  as  well  as  a  deodorant,  and  if 
applied  as  a  wash,  in  the  proportion  of  four  to  six  ounces 
of  the  powder  to  one  gallon  of  v/ater,  it  will  destroy  most 
forms  of  life.  In  many  cases  this  a:;ent  is  inapplicable  on 
account  of  its  odor. 

Corrosive  sublimate  (Hg  01 2)  for  most  purposes  is  a  good 
disinfectant,  but  it  is  such  an  intense  poison  that  its  use  is 
dangerous  in  places  that  are  at  all  accessible  to  stock. 

For  the  disinfection  of  walls  in  stables  and  barns,  com- 
mon thin  white  wash  (Ca  OH)  is  admirably  adapted  if  made 
from  freshly-burned  quick  lime.  It  possesses  strong germi- 
cidal  powers,  increases  the  amount  of  light  in  the  barn,  is 
a  good  absorbent  of  odors,  and  is  exceedingly  cheap. 

Carbolic  acid,  creosote,  and  such  products,  while  excellent 
disinfectants,  cannot  well  be  used  on  account  of  their  odor, 
especially  in  factories. 

For  gutters,  drains,  and  waste  pipes  in  factories,  vitriol 
salts  (sulfates  of  copper,  iron  and  zinc)  are  sometimes  used. 
These  are  deodorants  as  well  as  disinfectants,  and  are  not 
so  objectionable  to  use  on  account  of  their  odor. 

These  suggestions  as  to  the  use  of  chemicals,  however, 
only  apply  to  extreme  cases  and  should  not  be  brought  into 
requisition  until  a  thorough  application  of  hot  water,  soap, 
a  little  soda,  and  the  scrubbing  brush  have  failed  to  do  their 


CHAPTER  V. 
RELATION  OF  DISEASE-BACTERIA  TO  MILK. 

PRACTICAL  experience  with  epidemic  disease  has  abun- 
dantly demonstrated  the  fact  that  milk  not  infrequently 
serves  as  a  vehicle  for  the  dissemination  o**  contagion.  At- 
tention has  been  prominently  called  to  this  relation  by 
Ernest  Hart,1  who  in  1880  compiled  statistical  evidence 
showing  the  numerous  outbreaks  of  various  contagious  dis- 
eases that  had  been  associated  with  milk  infection  up  to 
that  time.  Since  then,  further  compilations  have  been 
made  by  Freeman,9  and  also  by  Busey  and  Kober,3  who 
have  collected  the  data  with  reference  to  outbreaks  from 
1880  to  1899. 

These  statistics  indicate  the  relative  importance  of  milk 
as  a  factor  in  the  dissemination  of  disease. 

The  danger  from  this  source  is  much  intensified  for  the 
reason  that  milk,  generally  speaking,  is  consumed  in  a  raw 
state;  and  also  because  a  considerable  number  of  disease- 
producing  bacteria  are  able,  not  merely  to  exist,  but  actu- 
ally thrive  and  grow  in  milk,  even  though  the  normal 
milk  bacteria  are  also  present.  Moreover  the  recognition 
of  the  presence  of  such  pathogenic  forms  is  complicated 
by  the  fact  that  often  they  do  not  alter  the  appearance  of 

'  Hart,  Trans.  Int.  Med.  Cong.,  London,  1881,  4:491-544. 

»  Freeman,  Med.  Rec.,  March  28,  1896. 

* Busey  and  Kober,  Kept.  Health  Off.  of  Dist.  of  CpL,  Washington,  D.  C.,  1895, 
p.  299.  These  authors  present  in  this  report  an  elaborate  article  on  morbific  and 
infectious  milk,  giving  a  very  complete  bibliography  of  180  numbers.  They  ap- 
pend to  Hart's  list  (which  is  published  in  full)  additional  outbreaks  which  have 
occurred  since,  together  with  full  data  as  to  extent  of  epidemic,  circumstances 
governing  the  outbreak,  as  well  as  name  of  original  reporter  and  reference. 


Relation  of  Disease-Bacteria  to  Milk.  83 

the  milk  sufficiently  so  that  their  presence  can  be  detected 
by  a  physical  examination.  These  facts  which  have  been 
experimentally  determined,  coupled  with  the  numerous 
-clinical  cases  on  record,  make  a  strong  case  against  milk 
serving  as  an  agent  in  the  dissemination  of  disease. 

Origin  of  pathogenic  bacteria  in  milk.  Disease-produc- 
ing bacteria  may  be  grouped  with  reference  to  their  relation 
toward  milk  into  two  classes,  depending  upon  the  manner 
in  which  infection  occurs: 

Class  I.  Disease-producing  bacteria  capable  of  being 
transmitted  directly  from  a  diseased  animal  to  man  through 
the  medium  of  infected  milk. 

Class  II.  Bacteria  pathogenic  for  man  but  not  for  cattle 
which  are  capable  of  thriving  in  milk  after  it  is  drawn  from 
the  animal. 

In  the  first  group  the  disease  produced  by  the  specific 
organism  must  be  common  to  both  cattle  and  man.  The 
organism  must  live  a  parasitic  life  in  the  animal,  develop- 
ing in  the  udder,  and  so  infect  the  milk  supply.  It  may, 
of  course,  happen  that  diseases  toward  which  domestic  ani- 
mals alone  are  susceptible  may  be  spread  from  one  animal 
to  another  in  this  way  without  affecting  human  beings. 

In  the  second  group,  the  bacterial  species  lives  a  sapro- 
phytic  existence,  growing  in  milk,  if  it  happens  to  find  its 
way  therein.  In  such  cases  milk  indirectly  serves  as  an 
.agent  in  the  dissemination  of  disease,  by  giving  conditions 
favorable  to  the  growth  of  the  disease  germ. 

By  far  the  most  important  of  diseases  that  may  be  trans- 
mitted directly  from  animal  to  man  through  a  diseased 
milk  supply  is  tuberculosis,  but  in  addition  to  this,  foot 
and  mouth  disease  (aphthous  fever  in  children),  anthrax 
and  acute  enteric  troubles  have  also  been  traced  to  a  sim- 
ilar source  of  infection. 


84  Dairy  Bacteriology. 

The  most  important  specific  diseases  that  have  been  dis- 
seminated through  subsequent  pollution  of  the  milk  are 
typhoid  fever,  diphtheria,  scarlet  fever  and  cholera,  but,  of 
course,  the  possibility  exists  that  any  disease  germ  capable 
of  living  and  thriving  in  milk  may  be  spread  in  this  way. 
.In  addition  to  these  diseases  that  are  caused  by  the  intro- 
duction of  specific  organisms  (the  causal  organism  of  scar- 
let fever  has  not  yet  been  definitely  determined),  there  are 
a  large  number  of  more  or  less  illy-defined  troubles  of  an 
intestinal  character  that  occur  especially  in  infants  and 
young  children  that  are  undoubtedly  attributable  to  the 
activity  of  microorganisms  that  gain  access  to  milk  during 
and  subsequent  to  the  milking,  and  which  produce  changes 
in  milk  before  or  after  its  ingestion  that  result  in  the 
formation  of  toxic  products. 

DISEASES  TRANSMISSIBLE  FROM  ANIMAL  TO  MAN  THROUGH 
DISEASED   MILK. 

Tuberculosis.  In  view  of  the  wide-spread  distribution 
of  this  disease  in  both  the  human  and  the  bovine  race,  the 
relation  of  the  same  to  milk  supplies  is  a  question  of  great 
importance.  It  is  now  generally  admitted  that  the  differ- 
"ent  types  of  tubercular  disease  found  in  different  kinds  of 
animals  and  man  are  attributable  to  the  development  of 
the  same  organism,  Bacillus  tuberculosis,  although  there 
are  varieties  of  this  organism  found  in  different  species  of 
animals  that  are  sufficiently  distinct  to  permit  of  recogni- 
tion. 

The  question  of  prime  importance  is,  whether  the  bovine 
type  is  transmissible  to  the  human  or  not.  Artificial  in- 
oculation of  cattle  with  tuberculous  human  sputum  as  well 
as  pure  cultures  of  this  variety  show  that  the  human  type 


Relation  of  Disease-Bacteria  to  Milk.  85 

is  able  to  make  but  slight  headway  in  cattle.  This  would 
indicate  that  the  danger  of  cattle  acquiring  the  infection 
from  man  would  in  all  probability  be  very  slight,  but  these 
experiments  offer  no  answer  as  to  the  possibility  of  trans- 
mission from  the  bovine  to  the  human.  Manifestly  it  is 
impossible  to  solve  this  problem  by  direct  experiment  upon 
man  except  by  artificial  inoculation,  but  comparative  ex- 
periments upon  animals  throw  some  light  on  the  question. 

Theo.  Smith '  and  others2  have  made  parallel  experiments 
with  animals  such  'as  guinea  pigs,  rabbits  and  pigeons,  in- 
oculated with  both  bovine  and  human  cultures  of  this  or- 
ganism. The  results  obtained  in  the  case  of  all  animals 
tested  show  that  the  virulence  of  the  two  types  was  much 
different,  but  that  the  bovine  cultures  were  much  more  se- 
vere. While  of  course  this  does  not  prove  that  transmis- 
sion from  bovine  to  human  is  possible,  still  the  importance 
of  the  fact  must  not  be  overlooked. 

In  a  number  of  cases  record  of  accidental  infection  from 
cattle  to  man  has  been  noted.8  These  have  occurred  with 
persons  engaged  in  making  post-mortem  examinations  on 
tuberculous  animals,  and  the  tubercular  nature  of  the  wound 
was  proven  in  some  cases  by  excision  and  inoculation. 

In  addition  to  data  of  this  sort  that  is  practically  experi- 
mental in  character,  there  are  also  strong  clinical  reasons 
for  considering  that  infection  of  human  beings  may  occur 
through  the  medium  of  milk.  Naturally  such  infection 
should  produce  intestinal  tuberculosis,  and  it  is  noteworthy 
that  this  phase  of  the  disease  is  quite  common  in  children 

1  Smith,  Theo.,  Journ.  of  Expt.  Med.,  1898,  3:  451. 

2  Dmwiddie,  Bull.  57,  Ark.  Expt.  Stat.,  June,  1899;  Ravenel,  Univ.  of  Penn. 
Med.  Bull ,  Sept.  1901. 

3  Ravenel,  Journ.  of  Comp.  Med.  &  Vet.  Arch.,  Dec.  1897;  Hartzell,  Journ. 
Amer.  Med.  Ass'n,  April  16,  1893. 


86  Dairy  Bacteriology. 

especially  between  the  ages  of  two  and  five.1  It  is  difficult 
to  determine,  though,  whether  primary  infection  occurred 
through  the  intestine,  for,  usually,  other  organs  also  be- 
come involved.  In  a  considerable  number  of  cases  in  which 
tubercular  infection  by  the  most  common  channel,  inhala- 
tion, seems  to  be  excluded,  the  evidence  is  strong  that  the 
disease  was  contracted  through  the  medium  of  the  milk,  but 
it  is  always  very  difficult  to  exclude  the  possibility  of  pul- 
monary infection. 

Tuberculosis  as  a  bovine  disease  has  increased  rapidly 
during  recent  decades  throughout  many  portions  of  the 
world.  This  has  been  most  marked  in  dairy  regions.  Its 
extremely  insidious  nature  does  not  permit  of  an  early  rec- 
ognition by  physical  means,  and  it  was  not  until  the  intro- 
duction of  the  tuberculin  test  *  in  1892,  as  a  diagnostic  aid 
that  accurate  knowledge  of  its  distribution  was  possible. 
The  quite  general  introduction  of  this  test  in  many  regions 
has  revealed  an  alarmingly  large  percentage  of  animals  as 
affected.  In  Denmark  in  1894  over  forty  per  cent  were 
diagnosed  as  tubercular.  In  some  parts  of  Germany  almost 
as  bad  a  condition  has  been  revealed.  Slaughter-house  sta- 
tistics also  show  that  the  disease  has  increased  rapidly  since 
1890.  In  this  country  the  disease  on  the  average  is  much 
less  than  in  Europe  and  is  also  very  irregularly  distributed. 
In  herds  where  it  gained  a  foothold  some  years  a£O,  often 
the  majority  of  animals  are  frequently  infected;  many 

'  Stille,  Brit.  Med.  Journ.,  Aug.  19,  1899. 

2  This  test  is  made  by  injecting  into  the  animal  a  small  quantity  of  tuberculin, 
which  is  a  sterilized  glycerin  extract  of  cultures  of  the  tubercle  bacillus.  In  a 
tuberculous  animal,  even  in  the  very  earliest  phases  of  the  disease,  tuberculin 
causes  a  temporary  fever  that  lasts  for  a  few  hours.  By  taking  the  temperature 
a  number  of  times  before  and  after  injection  it  is  possible  to  readily  recognize 
any  febrile  condition.  A  positive  diagnosis  is  made  where  the  temperature  after 
inoculation  is  at  least  2.0°  F.  above  the  average  normal,  and  where  the  reaction 
fever  is  continued  for  a  period  of  some  hours. 


Relation  of  Disease-Bacteria  to  Milk.  87 

herds,  in  fact  the  great  majority,  are  wholly  free  from  all 
taint.  The  disease  has  undoubtedly  been  most  frequently 
introduced  through  the  purchase  of  apparently  healthy  but 
incipiently  affected  animals.  Consequently  in  the  older 
dairy  regions  where  stock  has  been  improved  the  most  by 
breeding,  more  of  the  disease  exists  than  among  the  western 
and  southern  cattle. 

Infectiousness  of  milk  of  reacting  animals.  Where  the 
disease  appears  in  the  udder  the  milk  almost  invariably  con- 
tains the  tubercle  organism.  Under  such  conditions  the 
appearance  of  the  milk  is  not  materially  altered  at  first, 


FIG.  21.    Side  view  of  a  tuberculous  udder,  showing  extent  of  swelling  in 
single  quarter. 

but  as  the  disease  progresses  the  percentage  of  fat  generally 
diminishes,  and  at  times  in  the  more  advanced  stages  where 
the  physical  condition  of  the  udder  is  changed  (Fig.  21), 


88  Dairy  Bacteriology. 

the  milk  may  become  "watery";  but  the  percentage  of 
animals  showing  such  udder  lesions  is  not  large,  usually 
not  more  than  a  few  per  cent. 

On  the  other  hand,  in  the  earlier  phases  of  the  disease, 
where  its  presence  has  been  recognized  solely  by  the  aid  of 
the  tuberculin  test,  before  there  are  any  recognizable  phys- 
ical symptoms  in  any  part  of  the  animal,  the  milk  is  gener- 
all}r  unaffected.  Between  these  extremes,  however,  is  found 
a  large  proportion  of  cases,  concerning  which  so  definite 
data  are  not  available.  The  results  of  investigators  on  this 
.point  are  conflicting  and  further  information  is  much  de- 
sired. Some  have  asserted  so  long  as  the  udder  itself  shows 
no  lesions  that  no  tubercle  bacilli  would  be  present,1  but 
the  findings  of  a  considerable  number  of  investigators*  in- 
dicate that  even  when  the  udder  is  apparently  not  diseased 
the  milk  may  contain  the  specific  organism  as  revealed  by 
inoculation  experiments  upon  animals.  In  some  cases, 
however,  it  has  been  demonstrated  by  post-mortem  exam- 
ination that  discoverable  udder  lesions  existed  that  were 
not  recognizable  before  autopsy  was  made.  In  the  experi- 
mental evidence  collected,  a  varying  percentage  of  reacting 
animals  were  found  that  gave  positive  results;  and  this 
number  wa?  generally  sufficient  to  indicate  that  the  danger 
of  using  milk  from  reacting  animals  was  considerable,  even 
though  apparently  no  disease  could  be  found  in  the  udder. 

The  infectiousness  of  milk  can  also  be  proven  by  the 
frequent  contraction  of  the  disease  in  other  animals,  such 
as  calves  and  pigs  which  may  be  fed  on  the  skim  milk.  The 
very  rapid  increase  of  the  disease  among  the  swine  of  Ger- 

1  Martin,  Brit.  Med.  Journ.  1895, 1:937;  Nocard,  Les  Tuberculoses  animates,  1895. 

a  Bang.  Schmidt's  Jahrb.,  235:22;  Hirschberger,  Arch.  f.  klin.  Medicin,  1889; 

Ernst,  Infectiousness  of  Milk,  1895;  Ravenel,  Bull.  75,  Penn.  Dept.  Agr,   1901. 


Relation  of  Disease-Bacteria  to  Milk.  89 

many  and  Denmark,1  and  the  frequently  reported  cases  of 
intestinal  infection  of  young  stock  also  attest  the  presence 
of  the  organism  in  milk. 

The  tubercle  bacillus  is  so  markedly  parasitic  in  its  hab- 
its, that,  under  ordinary  conditions,  it  is  incapable  of  grow- 
ing at  normal  air  temperatures.  There  is,  therefore,  no 
danger  of  the  germ  developing  in  milk  after  it  is  drawn 
from  the  animal,  unless  the  same  is  kept  at  practically  blood 
heat. 

Even  though  the  milk  of  some  reacting  animals  may  not 
contain  the  dangerous  organism  at  the  time  of  making  the 
test,  it  is  quite  impossible  to  foretell  how  long  it  will  re- 
main free.  As  the  disease  becomes  more  generalized,  or  if 
tuberculous  lesions  should  develop  in  the  udder,  the  milk 
may  pass  from  a  healthy  to  an  infectious  state. 

This  fact  makes  it  advisable  to  exclude  from  milk  sup- 
plies intended  for  human  use,  all  milk  of  animals  that  re- 
spond to  the  tuberculin  test;  or  at  least  to  treat  it  in  a 
manner  so  as  to  render  it  safe.  Whether  it  is  necessary  to 
do  this  or  not  if  the  milk  is  made  into  butter  or  cheese  is  a 
somewhat  different  question.  Exclusion  or  treatment  is 
rendered  more  imperative  in  milk  supplies,  because  the 
danger  is  greater*  with  children  with  whom  milk  is  often  a 
prominent  constituent  of  their  diet,  and  also  for  the  reason 
that  the  child  is  more  susceptible  to  intestinal  infection 
than  the  adult. 

The  danger  of  infection  is  much  lessened  in  butter  or 
cheese,  because  the  processes  of  manufacture  tend  to  dimin- 
ish the  number  of  organisms  originally  present  in  the  milk, 
and  inasmuch  as  no  growth  can  ordinarily  take  place  in  these 
products  the  danger  is  minimized.  Moreover,  the  fact  that 

i  Ostertag,  Milch  Zeit.,  22:672. 


90  Dairy  Bacteriology. 

these  foods  are  consumed  by  the  individual  in  smaller 
amounts  than  is  generally  the  case  where  milk  is  used,  and 
also  to  a  greater  extent  by  adults,  lessens  still  further  the 
danger  of  infection. 

Notwithstanding  this,  numerous  observers '  especially  in 
Germany  have  succeeded  in  finding  the  tubercle  bacillus  in 
market  butter,  but  this  fact  is  not  so  surprising  when  it  is 
remembered  that  a  very  large  fraction  of  their  cattle  show 
the  presence  of  the  disease  as  indicated  by  the  tuberculin 
test,  a  condition  that  does  not  obtain  in  any  large  section 
in  this  country. 

These  observations  on  the  presence  of  the  tubercle  bacil- 
lus in  butter  have  been  questioned  somewhat  of  late2  by 
the  determination  of  the  fact  that  butter  may  contain  an 
organism  that  possesses  the  property  of  being  stained  in 
the  same  way  as  the  tubercle  organism.  Differentiation 
between  the  two  forms  is  rendered  more  difficult  by  the 
fact  that  this  tubercle-like  organism  is  also  capable  of  pro- 
ducing in  animals  lesions  that  simulate  those  of  tubercu- 
losis, although  a  careful  examination  reveals  definite  differ- 
ences. Petri1  has  recently  determined  that  both  the  true 
tubercle  and  the  acid-resisting  butter  organism  may  be 
readily  found  in  market  butter. 

In  the  various  milk  products  it  has  been  experimentally 
determined  that  the  true  tubercle  bacillus  is  able  to  retain 
its  vitality  in  butter  for  a  number  of  months  and  in  cheese 
for  nearly  a  year. 

Treatment  of  milk  from  tuberculous  cows.  While  it  has 
been  shown  that  it  is  practically  impossible  to  foretell 
whether  the  milk  of  any  reacting  animal  actually  contains 

»  Obermiiller,  Hyg.  Rund ,  1897,  p.  712;  Petri,  Arb.  a.  d.  Kais.  Ges.  Amte,  1898, 
14: 1;  Hermann  und  Morgenrotb,  Hyg.  Rund.,  1898,  p.  217. 
•Rabinowitsch.  Zelt.  f.  Hyg.,  1897,  26: 90. 


Delation  of  Disease-Bacteria  to  Milk.  91 

tubercle  bacilli  or  not,  still  the  interests  of  public  health 
demand  that  no  milk  from  such  stock  be  used  for  human 
food  until  it  has  toen  rendered  safe  by  some  satisfactory 
treatment. 

1.  Heating.  By  far  the  best  treatment  that  can  be  given 
such  milk  is  to  heat  it.  The  temperature  at  which  this 
should  be  done  depends  upon  the  thermal  death  point  of 
the  tubercle  bacillus,  a  question  concerning  which  there 
has  been  conisderable  difference  of  opinion  until  very  re- 
cently. According  to  the  work  of  some  of  the  earlier  in- 
vestigators, the  tubercle  bacillus  in  its  vegetative  stage  is 
endowed  with  powers  of  resistance  greater  than  those  pos- 
sessed by  any  other  pathogenic  organism.  This  work  has 
not  been  substantiated  loy  the  most  recent  investigations 
on  this  subject.  In  determining  the  thermal  death  point 
of  this  organism,  as  of  any  other,  not  only  must  the  tern- 
perature  be  considered,  but  the  period  of  exposure  as  well, 
and  where  that  exposure  is  made  in  milk,  another  factor 
must  be  considered,  viz.,  the  presence  of  conditions  per- 
mitting of  the  formation  of  a  "  scalded  layer,"  for  as  Smith l 
first  pointed  out,  the  resistance  of  the  tubercle  organism 
toward  heat  is  greatly  increased  under  these  conditions. 
If  tuberculous  milk  is  heated  in  a  closed  receptacle  where 
this  scalded  membrane  cannot  be  produced,  the  tubercle 
bacillus  is  killed  at  140°  F.  in  15  to  20  minutes.  These 
results  which  were  first  determined  by  Smith,  under  labora- 
tory conditions,  and  confirmed  by  Russell  and  Hastings,9 
where  tuberculous  milk  was  heated  in  commercial  pasteur- 
izers, have  also  been  verified  by  Hesse.8  A  great  practical 
advantage  which  accrues  from  the  treatment  of  milk  at 

1  Th.  Smith.  Journ.  of  Expt.  Med.,  1899,  4:  217. 

2  Russell  and  Hastings,  18  Rept.  Wis.  Expt.  Stat.,  1901. 
8  Hesse,  Zeit.  f.  Hyg.,  1900,  34:346. 


92  Dairy  Bacteriology. 

140°  F.  is  that  the  natural  creaming  is  practically  unaf- 
fected. Of  course,  where  a  higher  ternperatu  re  is  employed, 
the  period  of  exposure  may  be  materially  lessened.  If 
milk  is  momentarily  heated  to  185°  F.,  it  is  sufficient  to 
destroy  the  vitality  of  the  tubercle  bacillus.  This  is  the 
plan  practiced  in  Denmark  where  all  skim  milk  and  whey 
must  be  heated  to  this  temperature  before  it  can  be  taken 
back  to  the  farm,  a  plan  which  is  designed  to  prevent  the 
dissemination  of  tuberculosis  and  foot  and  mouth  disease 
by  means  of  the  mixed  creamery  by-products.  This  course 
renders  it  possible  to  utilize  with  perfect  safety,  for  milk 
supplies,  the  milk  of  herds  reacting  to  the  tuberculin  test, 
and  as  butter  of  the  best  quality  can  be  made  from  cream 
or  milk  heated  to  even  high  temperatures,1  it  thus  becomes 
possible  to  prevent  with  slight  expense  what  would  other- 
wise entail  a  large  loss. 

2.  Dilution.  Another  method  that  has  been  suggested 
for  the  treatment  of  this  suspected  milk  is  dilution  with  a 
relatively  large  volume  ot  perfectly  healthy  milk.  It  is  a 
well  known  fact  that  to  produce  infection,  it  requires  the 
simultaneous  introduction  of  a  number  of  organisms,  and 
in  the  case  of  tuberculosis,  especially  that  produced  by  in- 
gestion,  this  number  is  thought  to  be  considerable.  Geb- 
hardt9  found  that  the  milk  of  tuberculous  cows,  which  was 
virulent  when  injected  by  itself  into  animals,  was  innocuous 
when  diluted  with  40  to  100  times  its  volume  of  healthy 
milk.  This  fact  is  hardly  to  be  relied  upon  in  practice,  un- 
less the  proportion  of  reacting  to  healthy  cows  is  positively 
known. 

It  has  also  been  claimed  in  the  centrifugal  separation  of 

1  Practically  all  of  the  finest  butter  made  in  Denmark  is  made  from  cream  that 
has  been  pasteurized  at  temperatures  varying  from  160°-185e  F. 
» Gebhardt,  Virch.  Arch.,  1890, 119:  12. 


Relation  of  Disease- Bacteria  to  Milk.          93 

cream  from  milk *  that  by  far  the  larger  number  of  tubercle 
bacilli  were,  thrown  out  with  the  separator  slime.  Moore* 
has  shown  that  the  tubercle  bacilli  in  an  artificially  in- 
fected milk  might  be  reduced  in  this  way,  so  as  to  be  no 
longer  microscopically  demonstrable,  yet  the  purification 
was  not  complete  enough  to  prevent  the  infection  of  ani- 
mals inoculated  with  the  milk. 

Another  way  to  exclude  all  possibility  of  tubercular  in- 
fection in  milk  supplies  is  to  reject  all  milk  from  reacting 
animals.  This  method  is  often  followed  where  pasteuriza- 
tion or  sterilization  is  not  desired.  In  dairies  where  the 
keeping  quality  is  dependent  upon  the  exclusion  of  bacteria 
by  stringent  conditions  as  to  milking  and  handling  ("sani- 
tary "  or  "  hygienic  "  milk),  the  tuberculin  test  is  frequently 
used  as  a  basis  to  insure  healthy  milk. 

Foot  and  mouth  disease.  The  widespread  extension  of 
this  disease  throughout  Europe  in  recent  years  has  given 
abundant  opportunity  to  show  that  while  it  is  distinctively 
an  animal  malady,  it  is  also  transmissible  to  man,  although 
the  disease  is  rarely  fatal.  The  causal  organism  has  not 
been  determined  with  certainty,  but  it  has  been  thoroughly 
proven  that  the  milk  of  affected  animals  possesses  infec- 
tious properties.8 

Hertwig  showed  the  direct  transmissibiiity  of  the  disease 
to  man  by  experiments  made  on  himself  and  others.  By 
ingesting  milk  from  an  affected  animal,  he  was  able  to 
produce  the  symptoms  of  the  disease,  the  mucous  mem- 
brane of  the  mouth  being  covered  with  the  small  vesicles 
that  characterize  the  malady.  It  has  also  been  shown  that 

J  Scheurlen,  Arb.  a.  d.  k.  Ges.  Amte,  1891,  7:  269;  Bang,  MUch  Zeit.,  1893,  p.  672 
2  Moore,  Year  Book  of  TJ.  S.  Dept.  Agr.,  1895,  p.  432. 

3Weigeland  Noack,  Jahres.  d.  Ges.  Med.,  1890,  p.  642;  Weissenberg,  Allg.  med. 
Cent.  Zeit.,  1890,  p.  1;  Baum,  Arch.  f.  Thierheilkunde,  1892,  18:16. 


94  Dairy  Bacteriology. 

the  virus  of  the  disease  may  he  conveyed  in  hutter.1  This 
disease  is  practically  unknown  in  this  country,  although 
widely  spread  in  Europe. 

There  are  a  number  of  other  hovine  diseases  such  as 
anthrax,2  lockjaw,3  and  hydrophobia4  in  which  it  has  been 
shown  that  the  virus  of  the  disease  is  at  times  to  be  found 
in  the  milk  supply,  but  in  most  cases  the  secretion  of  milk 
becomes  visibly  affected,  so  that  the  danger  of  using  such 
is  greatly  minimized. 

There  are  also  a  number  of  inflammatory  udder  troubles 
known  as  garget  or  mammitis  which  are  produced  by  bac- 
terial action.  In  most  of  these,  the  physical  appearance 
of  the  milk  is  so  changed,  and  often  pus  is  present  to  such 
a  degree  as  to  give  a  very  disagreeable  appearance  to  the 
milk.  Pus-forming  bacteria  (staphylococci  and  strepto- 
cocci) are  to  be  found  associated  with  such  troubles. 

DISEASES  TRANSMISSIBLE  TO  MAN  THROUGH  INFECTION  OF 
MILK    AFTER  WITHDRAWAL. 

Milk  is  so  well  adapted  to  the  development  of  bacteria 
in  general,  that  it  is  not  surprising  to  find  it  a  suitable 
medium  for  the  growth  of  many  pathogenic  species.  While 
this  statement  applies  primarily  to  milk  in  a  sterile  condi- 
tion, yet  in  some  cases,  disease-producing  bacteria  are  even 
able  to  grow  in  raw  milk  in  competition  with  the  normal 
milk  bacteria,  so  that  even  a  slight  contamination  may 
suffice  to  produce  infection. 

The  diseases  that  are  most  frequently  disseminated  in 

i  Schneider,  Munch,  med.  Wochenschr.,  1893,  No.  27;  Frohner,  Ziet.  L  Fleisch 
u.  Milchhygiene,  1891,  p.  66. 
"Feser,  Deutsche  Zeit.  f.  Thiermed.,  1880,  6:166. 
•Nocard,  Bull.  Gen.,  1885,  p.  54. 
«  Deutsche  Viertelsjahr.  f.  offentL  Gesundheitspflege,  1890,  20:444. 


Relation  of  Disease- Bacteria  to  Milk.  95 

this  way  are  typhoid  fever,  diphtheria,  scarlet  fever  and 
cholera,  together  with  the  various  illy-defined  intestinal 
troubles  of  a  toxic  character  that  occur  in  children,  es- 
pecially under  the  name  of  cholera  infantum,  summer 
complaint,  etc. 

Diseases  of  this  class  are  not  derived  directly  from  ani- 
mals because  cattle  are  not  susceptible  to  the  same. 

Modes  of  infection.  In  a  variety  of  ways,  however,  the 
milk  may  be  subject  to  contaminating  influences  after  it 
is  drawn  from  the  animal,  and  so  give  opportunity  for  the 
deyelopment  of  disease-producing  bacteria.  The  more  im- 
portant methods  of  infection  are  as  follows: 

1.  Infection  directly  from  a  pre-existing  case  of  disease  on 
premises.     Quite  frequently  a  person  in  the  early  stage  of 
a  diseased  condition  may  continue  at  his  usual  vocation  as 
helper  in  the  barn  or  dairy,  and  so  give  opportunity  for 
direct  infection  to  occur.     In  the  so-called  cases  of  "  walk- 
ing typhoid,"  this  danger  is  emphasized.     Again  during 
the  period  of  convalescence,  a  similar  opportunity  exists 
for  direct  infection.     This   method  functions   more   fre- 
quently in  scarlet  fever  than  in  typhoid.     In  some  cases 
infection  has  been  traced  to  storage  of  the  milk  in  rooms 
in  the  house  where  it  became  polluted  directly  by  the 
•emanations  of  the  patient.1    Among  the  dwellings  of  the 
lower  classes  where  a  single  room  has  to  be  used  in  com- 
mon this  source  of  infection  has  been  most  frequently  ob- 
served. 

2.  Infection  through  the  medium  of  another  person.    Not 
infrequently  another  individual  may  serve  in  the  capacity 
of  nurse  or  attendant  to  a  sick  person,  and  also  assist  in 
the  handling  of  the  milk,  either  in  milking  the  animals  or 

1  E.  Roth,  Deutsche  Vierteljahresschr.  f.  oflentl.  Gesundheitspfl.,  1890,  22:238. 


96  Dairy  Bacteriology. 

caring  for  the  milk  after  it  has  been  drawn.  Busey  and 
Kober  report  twenty-one  outbreaks  of  typhoid  fever  in 
which  dairy  employees  also  acted  in  the  capacity  of  nurses. 

3.  Pollution  of  milk  utensils.    The  most  frequent  method 
of  infection  of  cans,  pails,  etc.,  is  in  cleaning  them  with 
water  that  may  be  polluted  with  disease  organisms.    Often 
wells  may  be  contaminated  with  diseased  matter  of  intes- 
tinal origin,  as  in  typhoid  fever,  and  the  use  of  water  at 
normal  temperatures,  or  even  in  a  luke-warm  condition, 
give  conditions  permitting  of  infection.     Intentional  adul- 
teration of  milk  with  water  inadvertently  taken  from  pol- 
luted sources  has  caused  quite  a  number  of  typhoid  out- 
breaks.1    Sedgwick  and  Chapin  *  found  in  the  Springfield, 
Mass.,  epidemic  of  typhoid  that  the  milk  cans  were  placed 
in  a  well  to  cool  the  milk,  and  it  was  subsequently  shown 
that  the  well  was  polluted  with  typhoid  fecal  matte*. 

4.  Pollution  of  udder  of  animal  by  leading  in  infected 
water,  or  by  washing  same  with   contaminated  water. 
This  method  of  infection  would  only  be  likely  to  occur  in 
case  of  typhoid.     An  outbreak  at  the  University  of  Vir- 
ginia in  18933  was  ascribed  to  the  latter  cause. 

5.  Pollution    of  creamery  by-products,   skim-milk,  etc. 
Where   the  milk  supply  of  one  patron  becomes  infected 
with  pathogenic  bacteria,  it  is  possible  that  disease  may  be 
disseminated  through  the  medium  of  the  creamery,  the  in- 
fective agent  remaining  in  the  skim  milk  after  separation 
and  so  polluting  the  mixed  supply.     This  condition  is  more 
likely  to  prevail  with  typhoid  because  of  the  greater  toler- 
ance of  this  organism  for  acids  such  as  would  be  found  in  raw 

1  S.  W.  North,  London  Practitioner,  1889,  43:393. 

•Sedgwick  and  Chapin,  Boston  Med.  &  Surg.  Journ.,  1893, 129:485. 

»  Dabney,  Phila.  Med.  News,  1893,  68:630. 


Relation  of  Disease- Bacteria  to  Milk.          97 

milk.  The  outbreaks  at  Brandon,1  England,  in  1893,  Cas- 
tle Island,2  Ireland,  and  Marlboro,3  Mass.,  in  1894,  were 
traced  to  such  an  origin. 

While  most  outbreaks  of  disease  associated  with  a  pol- 
luted milk  supply  originate  in  the  use  of  the  milk  itself, 
yet  infected  milk  may  serve  to  cause  disease  even  when 
used  in  other  ways.  Several  outbreaks  of  typhoid  fever 
have  been  traced  to  the  use  of  ice  cream  where  there  were 
strong  reasons  for  believing  that  the  milk  used  in  the  man- 
ufacture of  the  product  was  polluted.4  Hankin5  details  a 
case  of  an  Indian  confection  made  largely  from  milk  that 
caused  a  typhoid  outbreak  in  a  British  regiment. 

Although  the  evidence  that  milk  may  not  infrequently 
serve  as  an  agent  in  spreading  disease  is  conclusive  enough 
to  satisfactorily  prove  the  proposition,  yet  it  should  be  borne 
in  mind  that  the  organism  of  any  specific  disease  in  ques- 
tion has  rarely  ever  been  found.  The  reasons  for  this  are 
quite  the  same  as  those  that  govern  the  situation  in  the 
case  of  polluted  waters,  except  that  the  difficulties  of  the 
problem  are  much  greater  in  the  case  of  milk  than  with 
water.  The  inability  to  readily  separate  the  typhoid  germ, 
for  instance,  from  thetiolon  bacillus,  an  organism  frequently 
found  in  milk,  presents  technical  difficulties  not  easily  over- 
come. The  most  potent  reason  of  failure  to  find  disease 
bacteria  is  the  fact  that  infection  in  any  case  must  occur 
sometime  previous  to  the  appearance  of  the  outbreak.  Not 
only  is  there  the  usual  period  of  incubation,  but  it  rarely 
happens  that  an  outbreak  is  investigated  until  a  number  of 

1  Welphy,  London  Lancet,  1804,  2:1085. 

2  Brit.  Med.  Journ.,  1894,  1:815. 

3  Mass.  Bd.  Health  Kept.,  1894,  p.  765. 

4  Turner,  London  Practitioner,  1892,  49:141;  Munro,  Brit.  Med.  Journ.,  1894,  2:829.. 
» Hankin,  Brit.  Med.  Journ.,  1894,  2:613. 

7 


98  Dairy  Bacteriology. 

cases  have  occurred.     In  this  interim  the  original  cause  of 
infection  may  have  ceased  to  be  operative. 

Typhoid  fever.  With  reference  to  the  diseases  likely  to 
to  be  disseminated  through  the  medium  of  milk,  infected 
after  being  drawn  from  the  animal,  typhoid  fever  is  the 
most  important.  The  reason  for  this  is  due  (1)  to  the  wide 
spread  distribution  of  the  disease;  (2)  to  the  fact  that  the 
typhoid  bacillus  is  one  that  is  capable  of  withstanding 
considerable  amounts  of  acid,  and  consequently  finds  even 
in  raw  milk  containing  the  normal  lactic  acid  ba'cteria  con- 
ditions favorable  for  its  growth.1  Ability  to  grow  under 
these  conditions  can  be  shown  not  only  experimentally,  but 
there  is  abundant  clinical  evidence  that  even  a  slight  infec- 
tion often  causes  extensive  outbreaks,  as  in  the  Stamford, 
Conn.,  outbreak  in  1895  where  386  cases  developed  in  a  few 
weeks,  97  per  cent  of  which  occurred  on  the  route  of  one 
milkman.  In  this  case  the- milk  cans  were  thoroughly  and 
properly  cleaned,  but  were  rinsed  out  with  cold  water  from 
a  shallow  well  that  was  found  to  be  polluted. 

The  most  common  mode  of  pollution  of  milk  with  typhoid 
organisms  is  where  the  milk  utensils  are  infected  in  one 
way  or  another.  Generally,  this  arises  from  the  use  of  pol- 
luted water  in  cleansing  the  vessels  or  in  intentional  water- 
ing of  the  milk.  Second  in  importance  is  the  carrying  of 
infection  by  persons  serving  in  the  dual  capacity  of  nurse 
and  dairy  attendant. 

Cholera.  This  germ  does  not  find  milk  so  favorable  a 
nutrient  medium  as  the  typhoid  organism,  because  it  is 
much  more  sensitive  toward  the  action  of  acids.  Kitasato* 

1  Heim  (Arb.  a.  d.  Kais.  Gesundheitsamte,  1889,  5:303)  finds  it  capable  of  living 
from  20-30  days  in  milk. 
'Kitasato,  Arb.  a.  d.  Kais.  Gesundheitsamte,  1:470. 


Relation  of  Disease- Bacteria  to  Milk.          99 

found,  however,  that  it  could  live  in  raw  milk  from  one  to 
four  days,  depending  upon  the  amount  of  acid  present.  In 
boiled  or  sterilized  milk  it  grows  more  freely,  as  the  acid- 
producing  forms  are  thereby  eliminated.  In  butter  it  dies 
out  in  a  few  days  (4  to  5). 

On  account  of  the  above  relation  not  a  large  number  of 
cholera  outbreaks  have  been  traced  to  milk,  but  Simpson * 
records  a  very  striking  case  in  India  where  a  number  of 
sailors,  upon  reaching  port,  secured  a  quantity  of  milk.  Of 
the  crew  which  consumed  this,  every  one  was  taken  ill,  and 
four  out  of  ten  died,  while  those  who  did  not  partake  es- 
caped without  any  disease.  It  was  later  shown  that  the 
milk  was  adulterated  with  water  taken  from  an  open  pool 
in  a  cholera  infected  district. 

Diphtheria.  According  to  Klein  2  the  diphtheria  organ- 
ism is  capable  of  developing  in  animals,  attacking  among 
other  organs  the  udder,  and  so  infecting  the  milk;  but 
Abbott8  or  Vladimirow4  failed  to  confirm  these  experi- 
ments. There  is  abundant  evidence  that  the  diphtheria 
organism  is  able  to  grow  luxuriantly  in  milk,  even  more 
so  in  raw  than  in  sterilized.5 

Infection  in  this  disease  is  more  frequently  attributable 
to  direct  infection  from  patient,  or  indirectly  through 
attendant. 

Scarlet  fever.  Although  it  is  more  difficult  to  study  the 
relation  of  this  disease  to  contaminated  milk  supplies,  be- 
cause the  causal  germ  of  scarlet  fever  is  not  yet  known, 
yet  the  origin  of  a  .consider  able  number  of  epidemics  has 

1  Simpson,  London  Practitioner,  1887,  39:144. 

»  Klein,  19  Loc.  Gov't.  Bd.  (Gt.  Brit.)  1889, 167. 

•Abbott,  Vet.  Mag.  1:  17. 

*  Vladimirow,  Arch.  Sci.  biol.  Inst.  Med.,  St.  Petersburg,  1892,  p.  84. 

*Schottelius  and  Ellerhorst,  Milch  Zeit.,  1897,  pp.  40  and  73. 


100  Dairy  Bacteriology. 

been  traced  to  polluted  milk  supplies.  An  outbreak  oc- 
curred at  East  Orange,1  New  Jersey,  a  few  years  ago  in 
wbich  from  one  to  four  cases  developed  in  each  of  sixteen 
families.  The  cause  of  the  outbreak  was  traced  to  the  son 
of  the  milkman  who,  soon  after  convalescence  from  an  at- 
tack of  the  disease,  resumed  his  work  as  milker. 

Diarrhoeal  diseases.  Milk  not  infrequently  acquires  the 
property  of  producing  diseases  of  the  digestive  tract  by 
reason  of  the  development  of  various  bacteria  that  form 
more  or  less  poisonous  by-products.  These  troubles  occur 
most  frequently  during  the  summer  months,  especially  with 
infants  and  children,  as  in  cholera  infantum  and  summer 
complaint.  The  higher  mortality  of  bottle-fed  infants2  in 
comparison  with  those  that  are  nursed  directly  is  explicable 
alone  on  the  theory  that  cows'  milk  is  the  carrier  of  the 
infection,  because  in  many  cases  it  is  not  consumed  until 
there  has  been  ample  time  for  the  development  of  organisms 
in  it.  As  a  cause  of  sickness  and  death  these  diseases  ex- 
ceed in  importance  all  other  specific  diseases  previously 
referred  to. 

The  cause  of  these  troubles  is  not  to  be  ascribed  to  any 
specific  kind  of  bacteria,  but  there  are  undoubtedly  a  large 
number  of  organisms  which  are  able  to  develop  toxic  sub- 
stances in  food  products,  especially  in  milk.  In  some  cases 
it  appears  that  the  development  of  the  poisonous  products 
takes  place  in  the  intestines  after  the  food  is  ingested.  The 
origin  of  these  bacteria  is  in  all  probability  due  to  the  in- 
troduction of  dirt  and  filth  that  find  their  way  into  the 
milk  at  the  time  of  milking.  Fliigge3  has  pointed  out  the 

i  Boston  Med.  Journ.,  1897,  136:  44. 
a  Baginsky,  Hyg.  Rund.,  1895,  p.  176. 
•Flttgge,  Zeit.  f.  Hyg.,  17: 272. 


Relation  of  Disease-Bacteria  to  Milk.        101 

fact  that  certain  peptonizing  species  which  are  frequently 
found  in  milk  possess  a  toxic  property  for  lower  animals. 
Ptomaine  poisoning.  Many  cases  of  poisoning  from  food 
products  are  also  reported  with  adults.  These  are  due  to 
the  formation  of  various  toxic  products,  generally  pto- 
maines, that  are  produced  as  a  result  of  infection  of  foods 
by  different  bacteria.  One  of  these  substances,  tyrotoxicon, 
was  isolated  by  Vaughan J  from  cheese  and  various  other 
products  of  milk,  and  found  to  possess  the  propert}'  of  pro- 
ducing ^symptoms  of  poisoning  similar  to  those  that  are 
noted  in  such  cases.  He  attributes  the  production  of  this 
toxic  effect  to  the  decomposition  of  the  elements  in  the 
milk  induced  by  putrefactive  forms  of  bacteria  that  develop 
where  milk  is  improperly  kept.2  Often  outbreaks  of  this 
character3  assume  the  proportions  of  an  epidemic,  where  a 
large  number  of  persons  use  the  tainted  food. 

>Zeit.  f.  physiol.  Chemie,  10:146;  9  Intern.  Hyg.  Cong.  (London),  1891,  p.  118. 
a  Vaughan  and  Perkins,  Arch.  f.  Hyg.,  27:308. 

3  Newton  and  Wallace  (Phila.  Med.  News,  1887,  50:570)  report  three  outbreaks  at 
Long  Branch,  N.  J.,  two  of  which  occurred  in  summer  hotels. 


CHAPTER  VI. 

PRESERVATION   OF  MILK  FOR  COMMERCIAL 
PURPOSES. 

IN  Chapter  III  it  has  been  shown  how  milk  becomes 
contaminated  with  various  kinds  of  bacteria  which  find  in 
this  liquid  most  favorable  conditions  for  development.  The 
result  of  this  contamination  is  that  the  period  during  which 
milk  has  a  commercial  value  for  food  purposes,  either  in 
the  form  of  milk,  or  milk  products,  such  as  butter  and 
cheese,  is  greatly  lessened,  thereby  causing  losses  of  consid- 
erable economic  importance. 

Moreover,  it  has  been  further  shown  (Chapter  Y)  that 
this  food  product  which  is  so  admirably  adapted  to  serve 
as  food  may  become  infected  with  disease-producing  organ- 
isms, and  so  be  the  means  of  disseminating  contagion. 

From  these  two  points  of  view,  therefore, 

1.  the  economic,  as  shown  by  the  keeping  quality  of  the 
milk,  and 

2.  the  hygienic,  as  shown  by  its  possibility  to  spread  dis- 
ease, it  is  highly  important  that  means  should  be  adopted, 
if  possible,  that  will  result  in  improving  the  keeping  qual- 
ity so  as  to  diminish  these  losses,  and  at  the  same  time  in- 
sure freedom  from  bacteria  capable  of  developing  disease. 
Inasmuch  as  the  fermentative  changes  which  ordinarily 
occur,  and  which  lessen  the  commercial  life  of  the  milk  as 
a  food,  depend  entirely  on  the  development  of  living  or- 
ganisms that  may  find  their  way  into  the  milk,  an  improve- 
ment in  the  condition  of  the  milk  may  be  secured  (1)  by 
excluding  bacterial  life  so  far  as  practicable,  at  the  time 


Preservation  of  Milk.  103 

the  milk  is  drawn,  and  subsequently  holding  the  milk  at 
temperatures  unfavorable  to  the  multiplication  of  the  rela- 
tively few  organisms  that  do  gain  access;  or  (2)  by  remov- 
ing those  organisms  wholly  or  in  part  after  they  have  once 
gained  access  to  the  milk.  If  all  are  not  eliminated,  it 
then  becomes  necessary  to  keep  the  milk  under  such  con- 
ditions as  to  check  the  growth  of  those  which  are  not  re- 
moved. 

Preservation  by  exclusion.  The  first  method  is  followed 
in  many  dairies  that  supply  high-grade  milk  for  city  deliv- 
ery. The  so-called  "  sanitary  "  or  u  hygienic  "  milk  is  usu- 
ally a  milk  that  has  been  handled  in  such  a  way  as  to  pre- 
vent the  introduction  of  most  bacteria  that  under  ordinary 
conditions  would  find  their  way  into  the  same.  The  merits 
of  a  milk  of  this  character  in  comparison  with  one  pre- 
served by  means  of  other  methods,  as  pasteurization,  is 
a  question  concerning  which  there  has  been  much  discus- 
sion. When  it  is  considered,  as  will  be  shown  later,  that, 
methods  of  preservation  can  be  successfully  applied  thafc 
will  not  apparently  change  the  chemical  and  physical  prop- 
erties of  milk,  it  is  an  open  question  that  must  be  decided 
in  each  case  whether  exclusion  of  bacteria  can  be  as  eco- 
nomically and  efficiently  performed  as  the  destruction  of 
the  living  organisms  by  heat.  Certain  it  is  that  the  pro- 
cess of  exclusion  must  be  confined  to  dairies  that  are  under 
individual  control.  The  impossibility  of  exercising  ade- 
quate control  with  reference  to  the  milking  process  and  the 
care  of  the  milk  immediately  thereafter,  when  the  same  is 
produced  on  different  farms,  is  evident. 

In  enhancing  the  keeping  quality  of  milk  by  removing 
the  bacteria  it  is  necessary  to  do  so  without  in  any  way 
materially  interfering  with  the  nutritive  qualities  of  the 


lOi  Dairy  Bacteriology. 

fluid.  The  different  methods  that  have  been  proposed  to 
accomplish  this  result  depend  upon  the  removal  of  the  con- 
tained organisms  by  mechanical  means,  or  their  destruc- 
tion in  the  milk  by  means  of  either  chemical  or  physical 
processes.  In  removing  the  bacteria  two  means  have  been 
more  or  less  extensively  employed,  as  filtration  and  cen- 
trifugal force. 

Filtration  Of  milk.  Straining  milk  through  cloth  or 
wire  strainers  has  always  been  used  as  a  means  of  cleaning 
milk  from  dirt  and  foreign  matter;  but  it  is  quite  evident 
that  the  removal  of  such  material  can  only  diminish  the 
germ  content  of  the  milk  to  the  extent  that  bacteria  would 
adhere  to  such  coarse  particles.  The  individual  organisms 
floating  in  milk  are  capable  of  passing  the  finest  strainer, 
and  consequently,  such  processes  are  more  accurately  meth- 
ods of  cleaning  and  purifying  the  milk  rather  than  meth- 
ods that  enhance  the  keeping  quality. 

Along  somewhat  similar  lines  are  the  various  methods 
of  filtration  that  have  been  devised.  The  use  of  germ- 
proof  filters,  such  as  the  Pasteur  or  Berkefeld  type,  are 
inadmissible  with  milk,  because  the  pores  of  these  filters 
are  so  fine  as  to  hold  back  practically  all  suspended  matter, 
fat  and  casein  as  well  as  the  bacteria. 

For  a  number  of  years,  gravel,  sand  or  quartz  filters 
have  been  employed  for  the  double  purpose  of  cleaning 
milk  and  preserving^  it.  Several  different  types  of  these 
filters  are  or  have  been  in  use.  The  most  satisfactory  are 
built  in  several  sections  so  as  to  permit  of  read}r  cleansing, 
a  process  which  must  be  most  thoroughly  carried  out  with 
apparatus  of  this  kind.  Bolle  of  Berlin  washes  his  filters 
first  with  boiling  water,  then  dilute  hydrochloric  acid,  and 
finally,  with  water  until  all  trace  of  acid  is  removed.  The 


Preservation  of  Milk.  105 

Copenhagen  Dairy  Co.  sterilize  their  gravel  filters  by  high 
heat.  In  other  cases  lime  water  is  used  in  cleansing.  Fil- 
ters of  this  type  remove  practically  all  dirt  and  a  consider- 
able proportion  of  the  contained  bacteria,  but  they  are  in- 
tended more  to  clean  the  milk  than  enhance  its  keeping 
quality.  Woodfiber  (cellulose)  has  also  been  tested  as  a 
filtering  substance  with  success. 

Centrifugal  cleaning:  of  milk.  The  familiar  coating  of 
slime  and  dirt  that  collects  on  the  inner  face  of  the  sepa- 
rator bowl  shows  that  centrifugal  force  can  be  successfully 
used  in  cleaning  or  clarifying  milk.  While  the  ordinary 
types  of  cream  separators  are  able  to  remove  this  material 
in  a  satisfactory  way,  special  machines  have  been  devised 
for  this  particular  purpose.  A  bacteriological  examination 
of  separator  slime  shows  it  to  be  teeming  with  myriads  of 
organisms,  and  the  rapid  decomposition  which  it  undergoes 
is  evidence  of  its  high  germ  content,  but  there  is  practi- 
cally little  or  no  improvement  in  the  keeping  quality  of 
milk  that  has  been  treated  in  this  way.  This  is  in  part 
due  to  the  fact  that  bacteria  reproduce  so  rapidly  that  a 
marked  variation  in  numbers  is  soon  obscured  by  relatively 
more  rapid  growth.  Eckles  and  Barnes1  find  that  from 
37  to  56  per  cent  of  the  total  number  of  bacteria  present  in 
milk  are  thrown  out  lay  passing  milk  through  separators. 
Where  milk  is  cleaned  in  this  way,  the  cream  and  skim 
milk  is  generally  mixed  again  immediately,  but  the  pas- 
sage of  cream  through  the  separator  bowl  tends  to  break 
down-  the  size  of  the  normal  fat-globule  clusters  and  so 
lessen  the  consistency  of  the  product.  Such  a  diminution 
in  "  body  "  diminishes  materially  the  ease  with  which  cream 
can  be  whipped. 

i  Eckles  and  Barnes,  Bull.  No.  59  Iowa  Expt,  Stat.,  Aug.  1901. 


106  Dairy  Bacteriology. 

Chemical  preservatives.  Numerous  attempts  have  been 
made  to  find  some  chemical  substance  that  could  be  added 
to  milk  which  would  preserve  it  without  interfering  with 
its  nutritive  properties,  but  as  a  general  rule  a  substance 
that  is  toxic  enough  to  destro}"  or  inhibit  the  growth  of 
bacterial  life  exerts  a  prejudicial  effect  on  the  tissues  of  the 
body.  The  use  of  chemicals,  such  as  carbolic  acid,  mercury 
salts  and  mineral  acids,  that  are  able  to  entirely  destroy  all 
life,  is  of  course  excluded,  except  when  milk  is  preserved  for 
analytical  purposes;  but  a  number  of  milder  substances 
are  more  or  less  extensively  employed,  although  the  statutes 
of  practically  all  states  forbid  their  use. 

The  substances  so  used  may  be  grouped  in  two  classes: 

1.  Those  that  unite  chemically  with  certain  b3'-products 
of  bacterial  growth  to  form  inert  substances.     Thus  bi- 
carbonate of  soda  neutralizes  the  acid  in   souring  milkr 
although  it  does  not  destroy  the  lactic  acid  bacteria. 

2.  Those  that  act  directly  upon  the  bacteria  in  milk,  re- 
straining or  inhibiting  their  development.    The  substances- 
most  frequently  utilized  are  salicylic  acid,  formaldehyde 
and  boracic  acid.     These  are  nearly  always  sold  to  the 
milk  handler,  under  some  proprietary  name,   at  prices 
greatly  in  excess  of  what  the  crude  chemicals  could  be 
bought  for  in  the  open  market.     Formaldehyde  bas  been 
widely  advertised  of  late,  but  its  use  is  fraught  with  the 
greatest  danger,  for  it  practically  renders  insoluble  all  al- 
buminous matter  and  its  toxic  effect  is  greatly  increased  in 
larger  doses. 

These  substances  are  generally  used  by  milk  handlers 
who  know  nothing  of  their  poisonous  action,  and  although 
it  ma}7  be  possible  for  adults  to  withstand  their  use  in 
dilute  form,  without  serious  results,  yet  their  addition  to 


Preservation  of  Milk.  107 

general  milk  supplies  that  may  be  used  by  children  is  lit- 
tle short  of  criminal.  The  sale  of  these  preparations  for 
use  in  milk  finds  its  only  outlet  with  those  dairymen  who 
are  anxious  to  escape  the  exactions  that  must  be  met  by 
all  who  attempt  to  handle  milk  in  the  best  possible  man- 
ner. Farrington  has  suggested  a  simple  means  for  the 
detection  of  preservalin  (boracic  acid).1  When  this  sub- 
stance is  added  to  fresh  milk,  it  increases  the  acidity  of 
milk  without  affecting  its  taste.  As  normal  milk  tastes 
sour  when  it  contains  about  0.3  per  cent  lactic  acid,  a  milk 
that  tests  as  much  or  more  than  this  without  tasting  sour 
has  been  probably  treated  with  this  antiseptic  agent. 

Physical  methods  of  preservation.  Methods  based  upon 
the  application  of  physical  forces  are  less  likely  to  injure 
the  nutritive  value  of  milk,  and  are  consequently  more 
effective,  if  of  any  value  whatever.  A  number  of  methods 
have*  been  tried  more  or  less  thoroughly  in  an  experimental 
way  that  have  not  yet  been  reduced  to  a  practical  basis,  as 
electricity,  use  of  a  vacuum,  and  increased  pressure.2  Con- 
densation has  long  been  used  with  great  success,  but  in  this 
process  the  nature  of  the  milk  is  materially  changed.  The 
keeping  quality  in  condensed  milk  often  depends  upon  the 
action  of  another  principle,  viz.,  the  inhibition  of  bacterial 
growth  by  reason  of  the  concentration  of  the  medium. 
This  condition  is  reached  either  by  adding  sugar  and  so  in- 
creasing the  soluble  solids,  or  by  driving  off  the  water  by 
evaporation,  preferably  in  a  vacuum  pan.  Temperature 
changes  are,  however,  of  the  most  value  in  preserving  milk, 
for  by  a  variation  in  temperature  all  bacterial  growth  can 
be  brought  to  a  standstill,  and  under  proper  conditions 
thoroughly  destroyed. 

1  Farrington,  Journ.  Amer.  Chem.  Soc.,  Sept.,  1896. 
»Hite,  Bull.  58,  West  Va.  Expt.  Stat,  1899. 


108 


Dairy  Bacteriology. 


Use  of  low  temperatures.  The  effect  of  chilling  or 
rapid  cooling  on  the  keeping  quality  of  milk  is  well  known. 
When  the  temperature  of  milk  is  lowered  to  the  neighbor- 
hood of  45°  F.,  the  development  of  bacterial  life  is  so  slow 
as  to  materially  increase  the  period  that  milk  remains 
sweet.  Within  recent  years,  attempts  have  been  made  to 
preserve  milk  so  that  it  c'ould  be  shipped  long  distances  by 
freezing  the  product,  which  in  the  form  of  milk-ice  could 
be  held  for  an  indefinite  period  without  change.1  A  modi- 
fication of  this  process  known  as  Casse's  system  has  been 
tried  on  an  extensive  scale  in  Copenhagen  and  in  several 
places  in  Germany.  This  consists  of  adding  a  small  block 
of  milk-ice  (frozen  milk)  to  large  cans  of  milk  (one  part  to 
about  fifty  of  milk)  which  may  or  may  not  be  pasteur- 
ized.2 This  reduces  the 
temperature  so  that 
the  milk  remains  sweet 
considerably  longer. 
Such  a  process  permits 
of  the  shipment  of  milk 
for  long  distances  with 
safety.  It  is  reported 
that  London  receives 
milk  from  Denmark  and 
Sweden  that  is  treated 
in  this  waj7. 

Use  of  high  tempera- 

tiif  ae         TJ™4-    1  1  FIG.  22.    Microscopic  appearance  of  nor- 

tures.     Heat  has   long    ^  m|lk  showingpthe  fat.globules  aggre. 
been  used  as  a  preserv-    gated  in  clusters. 
ing  agent.      Milk  has  been  scalded   or  cooked  to  keep 
it  from  time   immemorial.     Heat  may  be  used  at   differ- 

i  Milch  Zeit,,  1895,  No.  9. 
a  Ibid.,  1897,  No.  83. 


Preservation  of  Milk. 


109 


ent  temperatures,  and  when  so  applied  exerts  a  varying 
effect,  depending  upon  temperature  employed.  All  meth- 
ods of  preservation  by  heat  rest,  however,  upon  the  applica- 
tion of  the  heat  under  the  following  conditions: 

1.  A  temperature   above  the  maximum  growing-point 
(105°-115°  F.)  and  below  the  thermal  death-point  (130°- 
140°  F.)  will  prevent  further  growth,  and  consequently 
fermentative  action. 

2.  A  temperature  above  the  thermal  death-point  destroys 
bacteria,  and  thereby  stops  all  changes.    This  temperature 
varies,  however,  with  the  condition  of  the  bacteria,  and  for 
spores  is  much  higher  than  for  vegetative  forms. 

Attempts  have  been  made  to  employ  the  first  principle 
in  shipping   milk  by  rail,  viz.,  prolonged  heating  above 

growing  temperature, 
but  when  milk  is  so 
heated,  its  physical  ap- 
pearance is  changed.1 
The  methods  of  heating 
most  satisfactorily  used 
are  known  as  steriliza- 
tion and  pasteurization, 
in  which  a  degree  of 
temperature  is  used  ap- 
proximating the  boiling 
and  scalding  points  re- 
spectively. 

Effect  of  heat  on  milk. 

When  milk  is  subjected 
to  the  action  of  heat,  a 
number  of  changes  in  its 
physical  and  chemical  properties  are  to  be  noted. 


FIG.  23.  Microscopic  appearance  of  milk 
heated  above  140°  F.,  showing  the  homo- 
geneous distribution  of  fat-globules.  The 
physical  change  noted  in  comparison  with 
Fig.  22  causes  the  diminished  consistency  of 
pasteurized  cream. 


i  Bernstein,  Milch  Zeit.,  1894,  pp.  184,  200. 


110  Dairy  Bacteriology. 

1.  Diminished  "body"   When  milk,  but  more  especially 
cream,  is  heated  to  140°  F.  or  above,  it  becomes  thinner  in 
consistency   or  "body,1'  a   condition   which  is. due  to  a 
change  in  the  grouping  of  the  fat  globules.     In  normal 
milk,  the  butter  fat  for  the  most  part  is  massed  in  micro- 
scopic clots  as  shown  in  Fig.  22.     When  exposed  to  a  tem- 
perature of  1400  F.  or  above,  these  fat-globule  clots  break 
down,  and  the  globules  become  homogeneously  distributed 
as  in  Fig.  23.   Under  these  conditions  milk  does  not  cream 
readily.     When  cream  itself  is  so  heated,  its  consistency  is 
materially  reduced,  giving  the  impression  that  it  contains 
a  less  per  cent  of  butter  fat.     These  changes  seriously  af- 
fect the  general  adoption  of  heat  as  a  means  of  preserving 
milk  for  ordinary  market  use,  but  fortunately  this  defect 
can  be  overcome. 

2.  Cooked  taste.     If  milk  is  heated  to  160°  F.,  it  acquires 
a  cooked  taste  that  becomes  more  pronounced  as  the  tem- 
perature is  further  raised.     Milk  so  heated  develops  on  its 
surface  a  pellicle  or  "  skin.1'     The  cause  of  this  change  in 
taste  is  not  well  known.     Usually  it  has  been  explained 
as  being  produced  by  changes  in  the  nitrogenous  elements 
in  the  milk,  particularly  in  the  albumen.   Recently,  Thoer- 
ner !  has  pointed  out  the  coincidence  that  exists  between 
the  appearance  of  a  cooked  taste  and  the  loss  of  certain 
gases  that  are  expelled  by  heating.     He  finds  that  the 
milk  heated  in  closed  vessels  from  which  the  gas  cannot 
escape  has  a  much  less  pronounced  cooked  flavor  than  if 
heated  in  an  open  vessel.     The  so-called  "  skin  "  on  the 
surface  of  heated  milk  is  not  formed  when  the  milk  is 
heated  in  a  tightly-closed  receptacle.     By  some 8  it  is  as- 
serted that  this  layer  is  composed  of  albumen,  but  there  is 

..  » Thoerner,  Chem.  Zeit.,  18:  &45. 
8  Snyder,  Chemistry  of  Dairying,  p.  59. 


Preservation  of  Milk.  Ill 

evidence  to  show  that  it  is  modified  casein  due  to  the  rapid 
evaporation  of  the  milk  serum  at  the  surface  of  the  milk. 

3.  Digestibility.  Considerable  difference  of  opinion 
has  existed  in  the  minds  of  medical  men  as  to  the  relative 
digestibility  of  raw  and  heated  milks.  A  considerable 
amount  of  experimental  work  has  been  done  by  making 
artificial  digestion  experiments  with  enzyms,  also  diges- 
tion experiments  with  animals,  and  in  a  few  cases  with 
children.  The  results  obtained  by  different  investigators 
are  quite  contradictory,  although  the  preponderance  of  evi- 
dence seems  to  be  in  favor  of  the  view  that  heating  does 
impair  the  digestibility  of  milk,  especially  if  the  tempera- 
ture attains  the  sterilizing  point.1  It  has  been  observed 
that  there  is  a  noteworthy  increase  in  amount  of  rickets,9 
scurvy  and  marasmus  in  children  where  highly-heated  milks 
are  employed.  These  objections  do  not  obtain  with  refer- 
ence to  milk  heated  to  moderate  temperatures,  as  in  pas- 
teurization, although  even  this  lower  temperature  lessens 
slightly  its  digestibility.  The  successful  use  of  pasteurized 
milks  in  children's  hospitals  is  evidence  of  its  usefulness. 

4-  Fermentative  changes.  The  normal  souring  change  in 
milk  is  due  to  the  predominance  of  the  lactic  acid  bacteria, 
but  as  these  organisms  as  a  class  do  not  possess  spores, 
they  are  readily  killed  when  heated  above  the  thermal  death- 
point  of  the  developing  cell.  The  destruction  of  the  lactic 
forms  leaves  the  spore-bearing  types  possessors  of  the  field, 
and  consequently  the  fermentative  changes  in  heated  milk 

1  Doane  and  Price  (Bull.  77,  M«d.  Expt.  Stat.,  Aug.  1901)  give  quite  a  full  resumS 
of  the  work  on  this  subject  in  connection  with  rather  extensive  experiments 
made  by  them  on  feeding  animals  with  raw,  pasteurized  and  sterilized  milks. 

a  Kickets  is  a  disease  in  which  the  bones  lack  sufficient  mineral  matter  to  give 
them  proper  firmness.  Marasmus  is  a  condition  in  which  the  ingested  food  seems 
to  fail  to  nourish  the  body  and  gradual  wasting  away  occurs. 


112  Dairy  Bacteriology. 

are  not  those  that  usually  occur,  but  are  characterized  by 
the  curdling  of  the  milk  from  the  action  of  rennet  enzyms. 
5.  Action  of  rennet.  Heating  milk  causes  the  soluble  lime 
salts  to  be  precipitated,  and  as  the  curdling  of  milk  by  ren- 
net (in  cheese-making)  is  dependent  upon  the  presence  of 
these  salts,  their  absence  in  heated  milks  greatly  retards 
the  action  of  rennet.  This  renders  it  difficult  to  utilize 
heated  milks  in  cheese-making  unless  the  soluble  lime  salts 
are  restored,  which  can  be  done  by  adding  solutions  of  cal- 
cium chlorid. 

Sterilization.  As  ordinarily  used  in  dairying,  steriliza- 
tion means  the  application  of  heat  at  temperatures  approxi- 
mating, if  not  exceeding,  212°  F.  It  does  not  necessarily 
imply  that  milk  so  treated  is  sterile,  i.  e.,  germ-free;  for, 
on  account  of  the  resistance  of  spores,  it  is  practically  im- 
possible to  destroy  entirely  all  these  hardy  forms.  If  milk 
is  heated  at  temperatures  above  the  boiling  point,  as  is 
done  where  steam  pressure  is  utilized,  it  can  be  ren- 
dered practically  germ-free.  Such  methods  are  employed 
where  it  is  designed  to  keep  milk  sweet  for  a  long  period 
of  time.  The  treatment  of  milk  by  sterilization  has  not 
met  with  any  general  favor  in  this  country,  although  it  has 
been  more  widely  introduced  abroad.  In  most  cases  the 
process  is  carried  out  after  the  milk  is  bottled;  and  consid- 
erable ingenuity  has  been  exercised  in  the  construction  of 
devices  which  will  permit  of  the  closure  of  the  bottles  after 
the  sterilizing  process  has  been  completed.  Milks  heated 
to  so  high  a  temperature  have  a  more  or  less  pronounced 
boiled  or  cooked  taste,  a  condition  that  does  not  meet  with 
general  favor  in  this  country.  The  apparatus  suitable  for 
this  purpose  must,  of  necessity,  be  so  constructed  as  to  with- 
stand steam  pressure,  and  consequently  is  considerably 


Preservation  of  Milk.  113 

.more  expensive  than  that  required  for  the  simpler  pasteur- 
izing process. 

Pasteurization.  In  this  method  the  degree  of  heat  used 
ranges  from  140°  to  185°  F.  and  the  application  is  made  for 
only  a  limited  length  of  time.  The  process  was  first  exten- 
sively used  by  Pasteur  (from  whom  it  derives  its  name)  in 
combating  various  maladies  of  beer  and  wine.  Its  import- 
ance as  a  means  of  increasing  the  keeping  quality  of  milk 
was  not  generally  recognized  until  a  few  years  ago;  but  the 
method  is  now  growing  rapidly  in  favor  as  a  means  of  pre- 
serving milk  for  commercial  purposes.  The  method  does 
not  destroy  all  germ-life  in  milk;  it  affects  only  those  or- 
ganisms that  are  in  a  growing,  vegetative  condition;  but  if 
the  same  is  quickly  cooled,  it  enhances  the  keeping  quality 
very  materially.  It  is  unfortunate  that  this  same  term  is 
used  in  connection  with  the  heating  of  cream  as  a  prepara- 
tory step  to  the  use  of  pure  cultures  in  cream-ripening  in 
butter-making.  The  objects  to  be  accomplished  vary  mate- 
rially and  the  details  of  the  two  processes  are  dso  quite  dif- 
erent.  The  experiments  of  Bitter 1  indicate  that  when  stored 
at  86°  F.,  properly  pasteurized  milk  will  remain  sweet  from 
six  to  eight  hours  longer  than  raw  milk;  at  77°  F.,  ten 
hours;  at  73°  F.,  twenty  hours;  and  at  58°  F.,  from  fifty  to 
seventy  hours.  This  enhances  the  keeping  quality  enough 
so  that  it  serves  all  practical  purposes. 

While  pasteurizing  can  be  performed  on  a  small  scale  by 
the  individual,  the  process  can  also  be  adapted  to  the  com- 
mercial treatment  of  large  quantities  of  milk.  The  appa- 
ratus necessary  for  this  purpose  is  not  nearly  so  expensive 
as  that  used  in  sterilizing,  a  factor  of  importance  when 
other  advantages  are  considered.  In  this  country  pasteur- 

'  Bitter,  Zeit.  f.  Hyg.,  1890,  17:272. 
8 


114:  Dairy  Bacteriology. 

ization  has  made  considerable  headway,  not  only  in  sup- 
plying a  milk  that  is  designed  to  serve  as  children's  food, 
but  even  for  general  purposes. 

Conditions  that  determine  pasteurizing  limits.  Consid- 
erable latitude  with  reference  to  temperature  limits  is  per- 
missible in  pasteurizing,  but  there  are  certain  conditions 
which  should  be  met,  and  these,  in  a  sense,  fix  the  limits 
employed.  They  are  as  follows: 

1.  Physical  requirement.     Inasmuch  as  it  is  undesirable 
to  have  any  material  change  in  taste  and  appearance  in 
pasteurized  milk  from  that  normally  found  in  the  raw  pro- 
duct, the  pasteurizing  temperature  should  be  limited  to  the 
degree  of  heat  that  can  safely  be  employed  without  any 
danger  of  imparting  a  cooked  or  scalded  flavor  to  milk.   If 
the  exposure  is  made  for  any  considerable  period  of  time, 
say  fifteen  to  twenty  minutes,  this  change  in  taste  appears 
to  be  quite  permanent  when  the  milk  is  heated  to  158°  F. 
This  condition,  therefore,  determines  the  maximum  limit 
that  should  be  used  in  pasteurizing,  if  one  is  to  avoid 
the  production  of  a  cooked  flavor.     Even  below  this  tem- 
perature a  slight  change  in  flavor  occurs,  although  it  dis- 
appears upon  chilling  the  milk-     Where  access  of  air  is 
excluded  during  heating,  this  cooked  taste  does  not  develop 
so  markedly. 

2.  Biological  requirement.     To  be  of  value  in  increasing 
the  keeping  quality  of  milk  and  to  insure  freedom  from 
disease  bacteria,  it  is  necessary,  in  all  cases,  to  exceed  the 
thermal  death-point  of  at  least  the  actively  developing  bac- 
teria in  the  milk.     For  most  bacteria  this  limit  is  constant 
and  quite  sharply  defined,  ranging  from  130°  to  140°  F. 
where  the  exposure  is  made  for  ten  minutes.     Where  ex- 
posed for  a  briefer  period  of  time,  the  temperature  limit  is 


Preservation  of  Milk.  115 

necessarily  higher.  The  organism  that  is  invested  with  most 
interest  in  this  connection  is  the  tubercle  bacillus.  On  ac- 
count of  its  more  or  less  frequent  occurrence  in  milk  (see 
p.  87)  and  its  reputed  high  powers  of  resistance,  it  may 
well  be  taken  as  a  standard  in  pasteurizing. 

Thermal  death  limits  of  tubercle  bacillus.  Concerning 
the  exact  temperature  at  which  this  germ  is  destroyed  there 
is  considerable  difference  of  opinion.  Part  of  this  arises 
from  the  inherent  difficulty  in  determining  exactly  when 
the  organism  is  killed  (due  to  its  failure  to  grow  readily  on 
artificial  media),  and  part  from  the  lack  of  uniform  condi- 
tions of  exposure.  The  standards  that  previously  have 
been  most  generally  accepted  are  those  of  De  Man,1  who 
found  that  thirty  minutes  exposure  at  149°  F.,  fifteen  min- 
utes at  155°  F.,  or  ten  minutes  at  167°  F.,  sufficed  to  de- 
stroy this  germ. 

More  recently  it  has  been  conclusively  proven,2  and  these 
results  confirmed  by  different  investigators,3  that  if  tuber- 
culous milk  is  heated  in  closed  receptacles  where  the 
scalded  surface  pellicle  does  not  form,  the  vitality  of  this 
disease  germ  is  destroyed  at  140°  F.  in  a  brief  period  (15  to  20 
minutes).  If  the  conditions  of  heating  are  such  that  the 
surface  of  the  milk  is  exposed  to  the  air,  the  resistance  of 
bacteria  is  greatly  increased.  When  heated  in  open  vessels 
Smith  found  that  the  tubercle  organism  was  not  killed  in 
some  cases  where  the  exposure  was  made  for  at  least  an 
hour.  Russell  and  Hastings 4  have  shown  an  instance  where 
the  thermal  death-point  of  a  micrococcus  isolated  from 
pasteurized  milk  was  increased  12.5°  F.,  by  heating  it  under 

»De  Man,  Arch.  f.  Hyg.,  1893,  18:133. 

aTh.  Smith,  Journ.  of  Expt.  Med.,  1899,  4: 217. 

•Russell  and  Hastings,  17  Kept.  Wis.  Expt.  Stat.,  1900,  p.  147. 

4  Russell  and  Hastings,  18  Kept.  Ibid.,  1901. 


116  Dairy  Bacteriology. 

conditions  that  permitted  of  the  formation  of  the  scalded 
layer.  This  is  a  point  of  great  practical  importance  in  the 
treatment  of  milk  and  necessitates  the  use  of  machinery 
that  will  prevent  the  formation  of  this  surface  film.  It 
follows,  therefore,  from  these  results  that  a  temperature  of 
140°  F.  can  be  used  if  advisable,  although  higher  temper- 
atures are  not  inadmissible. 

Sanitary  advantages  of  pasteurized  milk.  Not  only  does 
pasteurized  milk  keep  longer  and  is  also  free  from  specific 
disease  bacteria,  but  its  use  has  been  of  utmost  importance 
in  checking  infant  mortality  from  diarrhoeal  disturbances. 
This  is  shown  in  the  diminished  death  rate  in  children's 
hospitals,  and  is  again  exemplified  on  a  large  scale  in  the 
results  that  have  been  obtained  in  New  York  City  through 
the  liberality  of  Nathan  Strauss,  who,  for  several  years, 
has  furnished  the  poor  children  of  that  city  with  pasteurized 
milk.  Since  the  introduction  of  this  milk,  the  death  rate 
of  those  under  five  years  of  age  has  dropped  over  ten  per 
thousand  living  persons,  a  condition  which  is  explicable 
in  large  measure  to  the  use  of  a  relatively  germ-free  milk. 

Pasteurized  milk  should  be  consumed  within  twenty-four 
hours  if  it  is  used  by  children.  If  left  under  conditions 
favorable  to  germination,  bacterial  growth  will  go  on,  and 
it  has  been  shown  that  the  type  of  fermentation  produced 
may  sometimes  be  deleterious.1 

High  vs.  low  temperature  pasteurization.  The  limit 
which  has  been  most  generally  employed  has  been  the 
maximum  at  which  a  cooked  flavor  did  not  appear,  and  in 
practice  this  has  been  155°  F.  for  a  period  of  exposure  of 
twenty  minutes.  Under  these  conditions,  pasteurization 
is  efficiently  and  thoroughly  performed,  but  the  applica- 
tion of  this  degree  of  heat  to  milk  results  in  a  diminution 

» Fltlgge,  Zeit.  f.  Hyg.,  1894, 17: 272. 


Preservation  of  Milk.  117 

of  creaming  property,  and  especially  in  cream  of  a  marked 
decrease  in  thickness.  Both  of  these  conditions  seriously 
interfere  with  the  general  extension  of  pasteurization,  be- 
cause the  consumer  does  not  like  a  milk  on  which  the 
cream  does  not  rise  thoroughly.  The  reasons  which  under- 
lie this  physical  change  have  already  been  noted  (p.  109), 
and  it  should  be  further  observed  that  if  milk  is  not 
pasteurized  at  a  temperature  exceeding  140°  F.,  this 
change  in  condition  does  not  obtain.  Milk  heated  to  this 
temperature  satisfactorily  fills  the  biological  requirement,  in 
that  the  vegetating  forms  of  the  acid-producing  as  well  as 
the  disease  bacteria  are  destroyed.  The  consequence  is 
that  the  keeping  quality  of  such  milk  is  practically  as  good 
as  if  it  was  heated  to  a  temperature  of  155°  F.  The  appli- 
cation of  this  temperature  results  in  the  preparation  of  a 
milk  or  cream  that  more  closely  approximates  the  condi- 
tion of  the  normal  product,  while  at  the  same  time  such 
milk  possesses  practically  all  of  the  advantages  that  are 
found  in  that  heated  to  a  higher  temperature. 

Bacteriological  studies.  The  following  bacteriological 
studies  as  to  the  effect  which  a  variation  in  temperature 
exerts  on  bacterial  life  in  milk  are  of  importance  as  indi- 
cating the  proper  temperature  limits  to  be  selected.  In 
the  following  table  the  exposures  were  made  for  a  uniform 
period  (20  minutes): 

The  bacterial  content  of  milk  heated  at  different  temperatures. 

Number  of  bacteria  per  cc.  in  milk. 

45°C.          50°C.      55°C.  60°C.    65°C.  70°C. 

Unheated    113°F.        122°F.    131°F.  140°F.  149°F.  158°F. 

Series  I.    2,895,000 1,260,000  798,000  82,000    5,770  3,900 

Series  II.     750,000     665,000     262,400  201,000  950       700  705 

Series  III.  1,350,000  1,100,000     260.000  215,000  575       610  650 

Series  IV.  1,750,000 •       87,3ftO    4,000    3,500  3,600 


118  Dairy  Bacteriology. 

It  appears  from  these  results  that  the  most  marked  de- 
crease in  temperature  occurs  at  140°  F.  (60°  C.).  At  113°  F. 
(45°  C.)  no  marked  diminution  was  noted;  at  122°  F.  (50°  C.) 
many  cells  were  killed,  but  the  larger  part  of  them  were 
not  killed  until  a  temperature  of  140°  F.  (60°  C.)  was 
reached.  It  should  also  be  observed  that  an  increase  in 
heat  above  this  temperature  did  not  materially  diminish  the 
number  of  organisms  present,  indicating  that  those  forms 
remaining  were  in  a  spore  or  resistant  condition.  It  was 
noted,  however,  that  the  developing  colonies  grew  more 
slowly  in  the  plates  made  from  the  highly  heated  milk, 
showing  that  their  vitality  was  injured  to  a  greater  ex- 
tent. 

Applicability  to  general  use.  This  method  of  low  tem- 
perature pasteurization  has  now  been  tried  under  practical 
conditions  for  a  sufficient  period  to  determine  its  utility  as  a 
means  of  preserving  general  market  milk.  The  fact  that 
it  does  not  modify  in  any  essential  particular  the  normal 
characters  of  milk  is  a  point  much  in  its  favor.  Enhance- 
ment in  keeping  quality  and  freedom  from  disease  organ- 
isms are  factors  of  such  value  that  they  readily  commend 
such  milk  to  the  general  consumer.  With  the  improve- 
ments that  have  been  made  in  pasteurizing  machinery,  it 
is  now  possible  to  handle  considerable  quantities  and  so 
reduce  very  much  the  cost  of  treatment  per  gallon.  This 
method  is  especially  applicable  to  the  treatment  of  cream. 
The  extreme  perishable  nature  of  this  milk  product  makes 
it  imperative  that  it  should  be  handled  in  such  a  way  as  to 
check  as  far  as  possible  germ  growth,  and  this  can  be 
readily  accomplished  when  the  same  is  pasteurized  and 
kept  at  low  temperatures.  The  higher  intrinsic  value  of 
this  product  lessens  the  relative  cost  of  operation  per  unit 
of  volume. 


Preservation  of  Milk.  119 

Within  the  last  few  years  this  system  has  been  quite 
widely  introduced  into  a  number  of  cities,  and  the  results 
obtained  are,  on  the  whole,  successful.  As  a  system  for 
general  use  it  does  not  meet  with  nearly  as  much  opposition 
as  is  offered  to  the  use  of  the  higher  limits. 

One  marked  advantage  accruing  to  this  system  is  its 
general  applicability.  Milk  can  be  pasteurized  where  it  is 
produced  under  a  single  management  as  in  the  individual 
dairy,  or  the  product  of  several  patrons  can  be  treated  to- 
gether, as  in  a  factory.  The  fresher  and  better  the  milk  is, 
though,  the  more  suitable  it  is  for  pasteurizing.  Therefore, 
while  it  is  possible  to  somewhat  improve  milk  that  has 
been  collected  for  some  hours  (12  to  24)  if  it  is  properly 
pasteurized,  still  better  results  will  be  obtained  if  the  treat- 
ment is  given  nearer  the  animal.  Under  practical  condi- 
tions, however,  pasteurizing  near  the  place  of  consumption 
has  some  advantages  and  is  preferable,  if  it  is  possible  to 
transport  the  raw  material  quickly  from  the  place  of  pro- 
duction. 

For  the  preparation  of  high-grade  milk  supplies  it  may 
be  said  that  either  the  elimination  of  the  bacteria  by  pas- 
teurization, or  preventing  their  gaining  access  to  the  milk, 
as  in  sanitary  dairies,  is  the  most  feasible  and  successful 
way  to  deal  with  this  question. 

Restoration  of  "body"  of  pasteurized  cream.  The  ac- 
tion of  heat  causes  the  tiny  groupings  of  -fat  globules  in 
normal  milk  (Fig.  22)  to  break  up,  and  with  this  change, 
which  occurs  in  the  neighborhood  of  140°  F.,  the  consist- 
ency of  the  liquid  is  diminished,  notwithstanding  the  fact 
that  the  fat-content  remains  unchanged.  Babcock  and 
the  writer *  devised  the  following  "  cure  "  for  this  apparent 

1  Babcock  and  Russell,  Bull.  54,  Wis.  Expt.  Stat.,  also  13  Eept.  Wis.  Expt.  Stat. 
1896,  p.  81. 


120 


Da  iry  Ba cier ioloyy. 


defect.  If  a  strong  solution  of  cane  sugar  is  added  to 
freshly  slacked  lime  and  the  mixture  allowed  to  stand,  a 
clear  fluid  can  be  decanted  off.  The  addition  of  this  alka- 
line liquid,  which  is  called  u  viscogeu,"  to  pasteurized  cream 
in  proportions  of  about  one  part  of  sugar-lime  solution  to 
100  to  150  of  cream,  restores  the  consistencyof  the  cream,  as 
it  causes  the  fat  globules  to  cluster  together  in  small  groups. 
The  relative  viscosity  of  creams  can  easily  be  determined 
by  the  following  method  (Fig.  24): 


A  B 

FIG.  24.    Relative  consistency  of  pasteurized  cream  before  (A)  and  after  (B) 
treatment  with  viscogen  as  shown  by  rate  of  flow  down  inclined  glass  plate. 

Take  a  perfectly  clean  piece  of  glass  (plate  or  picture 
glass  is  preferable,  as  it  is  less  liable  to  be  wavy).    Drop  on 


Preservation  of  Milk.  121 

one  edge  two  or  three  drops,  of  cream  at  intervals  of  an 
inch  or  so.  Then  incline  piece  of  glass  at  such  an  angle 
as  to  cause  the  cream  to  flow  down  surface  of  glass.  The 
cream,  having  the  heavier  body  or  viscosity,  will  move  more 
slowly.  If  several  samples  of  each  cream  are  taken,  then 
the  aggregate  lengths  of  the  different  cream  paths  may  be 
taken,  thereby  eliminating  slight  differences  due  to  condi- 
tion of  glass. 

Pasteurizing  details.  While  the  pasteurizing  process  is 
exceedingly  simple,  yet,  in  order  to  secure  the  best  results, 
certain  conditions  must  be  rigidly  observed  in  the  treat- 
ment before  and  after  the  heating  process. 

It  is  important  to  select  the  best  possible  milk  for  pas- 
teurizing, for  if  the  milk  has  not  been  milked  under  clean 
conditions,  it  is  likely  to  be  rich  in  the  spore-bearing  bac- 
teria. Old  milk,  or  milk  that  has  not  been  kept  at  a  low 
temperature,  is  much  richer  in  germ-life  than  perfectl}r 
fresh  or  thoroughly  chilled  milk. 

The  true  standard  for  selecting  milk  for  pasteurization 
should  be  to  determine  the  actual  number  of  bacterial 
spores  that  are  able  to  resist  the  heating  process,  but  this 
method  is  impracticable  under  commercial  conditions. 

The  following  method,  while  only  approximate  in  its  re- 
sults, will  be  found  helpful:  Assuming  that  the  age  or 
treatment  of  the  milk  bears  a  certain  relation  to  the  pres- 
ence of  spores,  and  that  the  acid  increases  in  a  general  way 
with  an  increase  in  age  or  temperature,  the  amount  of  acid 
present  may  be  taken  as  an  approximate  index  of  the  suit- 
ability of  the  milk  for  pasteurizing  purposes.  Biological 
tests  were  carried  out  in  the  author's  laboratory  *  on  milks 
having  a  high  and  low  acid  content,  and  it  was  shown  that 

i  Siiockley,  Thesis,  Univ.  of  Wis.,  1896. 


122 


Dairy  Bacteriology. 


THERMAL 
DEATH  POINT 


MAXIMUM 
GROWTH  POINT 
BLOOJ)  HEAT- 


MINIMUM    

'GROWTH  POINT 


FIG.  25.  Diagram  showing  tem- 
perature changes  in  pasteurizing, 
and  the  relation  of  same  to  bac- 
terial growth. 

Shaded  zone  represents  limits 
of  bacterial  growth,  50°-109°  F. 
(10°-43°  C.),  the  intensity  of  shad- 
ing indicating  rapidity  of  develop- 
ment. The  solid  black  line  shows 
temperature  of  milk  during  the 
process.  The  necessity  for  rapid 
cooling  is  evider.t  as  the  milk  fal's 
in  temperature  to  that  of  growing 
zone. 


Preservation  of  Milk.  123 

the  milk  with  the  least  acid  was,  as  a  rule,  the  freest  from 
spore-bearing  bacteria. 

This  acid  determination  can  be  made  at  the  weigh-can 
by  employing  the  Farrington  alkaline  tablet  which  is  used 
in  cream-ripening.  Where  milk  is  pasteurized  under  gen- 
eral creamery  conditions,  none  should  be  used  containing 
more  than  0.2  per  cent  acidity.  If  only  perfectly  fresh  milk 
is  used,  the  amount  of  acid  will  generally  be  about  0.15  per 
cent  with  phenolphthalein  as  indicator. 

Emphasis  has  already  been  laid  on  the  selection  of  a 
proper  limit  of  pasteurizing  (p.  114).  It  should  be  kept 
constantly  in  mind  that  the  thermal  death-point  of  any 
organism  depends  not  alone  on  the  temperature  used,  but  on 
the  period  of  exposure.  With  the  limits  given,  140°  to  155° 
F.,  it  is  necessary  to  expose  the  milk  for  not  less  than  fif- 
teen minutes.  If  a  higher  heat  is  employed  (and  the 
cooked  flavor  disregarded)  the  period  of  exposure  may  be 
curtailed. 

Chilling  the  milk.  It  is  very  essential  in  pasteurizing 
that  the  heated  milk  be  immediately  chilled  in  order  to 
prevent  the  germination  of  the  resistant  spores,  for  if  ger- 
mination once  occurs,  growth  can  go  on  at  relatively  low 
temperatures. 

The  following  experiments  by  Marshall '  are  of  interest 
as  showing  the  influence  of  refrigeration  on  germination 
of  spores : 

Cultures  of  organisms  that  had  been  isolated  from  pas- 
teurized milk  were  inoculated  into  bouillon.  One  set  was 
left  to  grow  at  room  temperature,  another  was  pasteurized 
%and  allowed  to  stand  at  same  temperature,  while  another 
heated  set  was  kept  in  a  refrigerator.  The  unheated  cul- 

» Marshall,  Mich.  Expt.  Stat.,  Bull.  147,  p.  47. 


124  Dairy  Bacteriology. 

tares  at  room  temperature  showed  evidence  of  growth  in 
thirty  trials  in  an  average  of  26  hours;  29  heated  cultures 
at  room  temperature  all  developed  in  an  average  of  50 
hours,  while  the  heated  cultures  kept  in  refrigerator  showed 
no  growth  in  45  days  with  but  four  exceptions. 

After  the  milk  is  pasteurized,  it  must  of  necessity  be 
stored  and  handled  in  germ-free  receptacles.  All  utensils, 
such  as  cans,  dippers,  bottles,  etc.,  must  be  thoroughly 
sterilized.  For  this  purpose  a  sterilizing  oven  should  be 
had  which  is  fitted  with  steam.  Material  of  this  sort,  after 
being  thoroughly  cleansed,  should  be  steamed  for  one-half 
to  three-quarters  of  an  hour.  Sterilized  bottles  should  be 
kept  protected  from  dust  until  they  are  used. 

Bottling  and  handling:  the  product.  In  bottling  the 
product  it  is  necessary  to  keep  the  milk  protected  from  re- 
infection. It  may  be  bottled  from  a  large  can  with  a  bottom 
faucet,  or,  on  a  large  scale,  with  commercial  bottling  ma- 
chines that  fill  several  bottles  at  once.  If  "  viscogen  "  is 
added  to  restore  consistency  of  cream,  it  should  be  done 
before  bottling,  but  not  before  the  cream  is  thoroughly 
cooled.  The  best  bottles  for  the  purpose  are  those  that 
have  a  plain  pulp  cap.  All  metal  fastenings  or  stoppers 
are  dirt-catchers  and  are  likely  to  get  out  of  order.  It  is 
our  practice  to  heat  pulp  caps  in  paraffin,  thereby  render- 
ing them  more  pliable  and  at  the  same  time  sterilizing 
them.  Bottles  sealed  with  hot  caps  in  this  way  are  tightly 
closed. 

In  delivering  pasteurized  products,  it  is  always  neces- 
sary to  use  care  in  handling  to  prevent  the  cream  and  milk 
from  being  warmed  up,  and  thus  inciting  into  activity  the 
latent  spores. 


Preservation  of  Milk.  125 

Pasteurizing:  apparatus.  The  problems  to  be  solved  in 
the  pasteurization  of  milk  and  cream  designed  for  direct 
consumption  are  so  materially  different  from  those  where 
butter  is  to  be  made  that  the  type  of  machinery  best  adapted 
to  each  purpose  is  quite  different.  The  equipment  neces- 
sary for  the  first  purpose  may  be  divided  into  two  general 
classes: 

1.  Apparatus  of  limited  capacity  designed  for  family  use. 

2.  Apparatus  of  sufficient  capacity  to  pasteurize  on  a 
commercial  scale. 

Domestic  pasteurizers.  In  pasteurizing  milk  for  indi- 
vidual use,  it  is  not  desirable  to  treat  at  one  time  more  than 
will  be  consumed  in  one  day;  hence  an  apparatus  holding 
a  few  bottles  will  suffice.  In  this  case  the  treatment  can 
best  be  performed  in  the  bottle  itself,  thereby  lessening  the 
danger  of  infection.  Several  different  types  of  pasteurizers 
are  on  the  market;  but  special  apparatus  is  by  no  means 
necessary  for  the  purpose.  The  process  can  be  efficiently 
performed  by  any  one  with  the  addition  of  an  ordinary  dairy 
thermometer  to  the  common  utensils  found  in  the  kitchen. 
Fig.  26  indicates  a  simple  contrivance  that  can  be  readily 
arranged  for  this  purpose. 

The  following  suggestions  indicate  the  different  steps  of 
the  process: 

1.  Use  only  fresh  milk. 

2.  Place  milk  in  clean  bottles  or  fruit  cans,  filling  to  a 
uniform  level,  closing  bottles  tightly  with  a  cork  or  cover. 
If  pint  and  quart  cans  are  used  at  the  same  time,  an  inverted 
bowl  will  equalize  the  level.     Set  these  in  a  flat-bottomed 
tin  pail  and  fill  with  warm  water  to  same  level  as  milk. 
An  inverted  pie  tin  punched  with  holes  will  serve  as  a  stand 
on  which  to  place  the  bottles  during  the  heating  process. 


126 


Dairy  Bacteriology. 


3.  Heafc  water  in  pail  until  the  temperature  of  same 
reaches  155°  to  160°  F. ;  then  remove  from  source  of  direct 
heat,  cover  with  a  cloth  or  tin  cover,  and  allow  the  whole 


FIG.  26.    A  home-made  pasteurizer. 

to  stand  for  half  an  hour.  In  the  preparation  of  milk  for 
children,  it  is  not  advisable  to  use  the  low-temperature 
treatment  (140°  F.)  that  is  recommended  for  commercial 
city  delivery. 

4.  Remove  bottles  of  milk  and  cool  them  as  rapidly  as 
possible  without  danger  to  bottles  and  store  in  a  refriger- 
ator. 

Commercial  pasteurizers.  As  noted  before,  the  object 
in  commercial  pasteurization  depends  upon  whether  it  is 
desired  to  treat  milk  for  general  milk  supply  or  to  make 
into  butter.  The  ends  to  be  attained  are  so  widely  different 
that  it  naturally  follows  that  the  apparatus  best  suited  for 
the  respective  purposes  must  vary  considerably.  In  pas- 
teurizing milk  in  butter-making,  capacity  is  one  of  the 


Preservation  of  Milk.  127 

most  important  desiderata,  but  in  preparing  milk  for  human 
use,  fulfillment  of  sanitary  conditions  is  the  first  requisite. 
It  is  to  be  regretted  that  milk  dealers  so  frequently  lose 
sight  of  this  requirement  in  their  attempts  to  secure  appa- 
ratus that  will  handle  large  amounts  so  as  to  reduce  the 
cost  of  operation.  Pasteurizing  involves  considerable  time 
and  trouble,  and  it  is  better  not  to  have  the  milk  treated  at 
all  than  to  have  the  process  imperfectl}T  performed. 

The  various  types  of  machinery  that  have  been  suggested 
for  this  use  may  be  grouped  as  follows,  depending  upon 
their  method  of  operation:1 

1.  Continuous- flow  machines. 

2.  Intermittent  machines. 

The  continuous-flow  pasteurizers  were  originally  designed 
for  the  treatment  of  milk  and  cream  for  butter-making,  but 
in  many  cases  they  have  been  applied  to  the  preservation 
of  milk  for  direct  consumption.  The  difficulty  with  them 
is  not  that  the  milk  cannot  be  readily  heated  in  the  same, 
but  as  customarily  arranged  there  is  no  provision  for  the 
retention  of  the  milk  at  a  temperature  that  would  be  fatal 
to  the  organisms  in  the  same. 

Continuous-flow  pasteurizers.  Apparatus  of  this  class 
vary  much  in  detail,  but  all  possess  this  common  principle, 
that  the  milk  enters  the  machine  in  a  continuous  stream 
and  is  generally  discharged  in  the  same  way.  The  objec- 
tion to  this  type  of  apparatus  is  that  the  time  of  heating 
cannot  be  regulated  with  any  certainty,  although  the  tem- 
perature can  be  controlled  in  part  by  varying  the  speed  of 
flow.  Recent  tests  made  at  the  Wisconsin  Dairy  School 

1  For  a  more  detailed  description  of  pasteurizing  machinery,  reference  should 
be  made  to  Monrad's  Pasteurization  of  Milk,  or  Weigmann's  Conservierung  der 
Milch. 


128  Dairy  Bacteriology. 

on  a  machine  of  this  type  showed  that  it  took  only  one 
and  one-half  minutes  for  milk  to  pass  through  the  ap- 
paratus, although  it  was  claimed  that  it  was  in  the  machine 
for  a  period  of  ten  minutes.  Another  objection  is  that  in 
the  rapid  heating,  where  steam  is  employed,  the  proteids  of 
the  milk  scald  on  to  the  walls  of  the  pasteurizer. 

In  some  of  these  machines  (Thiel,  Kuehne,  Lawrence, 
De  Laval,  and  Hochmuth),  a  ribbed  surface  is  employed 
over  which  the  milk  flows,  while  the  opposite  surface  is 
heated  with  hot  water  or  steam.  Monrad,  Lefeldt  and 
Lentsch  employ  a  centrifugal  apparatus  in  which  a  thin 
layer  of  milk  is  heated  in  a  revolving  drum. 

In  the  Hill  and  Miller  pasteurizers  (both  American  ma- 
chines), the  milk  is  forced  in  a  thin  sheet  between  heated 
surfaces  and  overflows  at  the  top. 

One  of  the  most  economical  types  of  apparatus  is  the 
regenerator  type  (a  German  machine),  in  which  the  milk 
passes  over  the  heating  surface  in  a  thin  stream  and  then 
is  carried  back  over  the  incoming  cold  milk  so  that  the 
heated  liquid  is  partially  cooled  by  the  inflowing  fresh  milk. 

A  number  of  machines  have  been  constructed  on  the 
principle  of  a  reservoir  which  is  fed  by  a  constantly  flow- 
ing stream.  In  some  kinds  of  apparatus  of  this  type  no 
attempt  is  made  to  prevent  the  mixing  of  the  recently  in- 
troduced milk  with  that  which  has  been  partially  heated. 
The  pattern  for  this  reservoir  type  is  Fjord's  heater,  in 
which  the  milk  is  stirred  by  a  stirrer.  This  apparatus  was 
originally  designed  as  a  heater  for  milk  before  separation. 
Reid  was  the  first  to  introduce  this  type  of  machine  into 
America.  A  recently  devised  machine  of  this  type  (Pas- 
teur) has  been  tested  by  Lehmann,  who  found  that  it  was 
necessary  to  heat  the  milk  as  high  as  176°  to  185°  F.,  in 


Preservation  of  Milk.  129 

order  to  secure  satisfactory  results  on  the  bacterial  content 
of  the  cream. 

Tests  Of  apparatus.  But  few  of  the  continuous-flow 
type  of  machines  have  been  subjected  to  rigid  bacterio- 
logical control,  and  their  efficiency  is  questionable.  By 
their  use  it  may  be  possible  to  enhance  the  keeping  qual- 
ity of  milk  in  a  fairly  satisfactory  way,  and  yet  not  in- 
sure complete  freedom  from  disease-producing  bacteria. 
One  grave  defect  in  many  of  them  is  that  all  parts  of  the 
milk  are  not  heated  uniformly.  It  is  easily  possible  for  one 
part  to  be  over-heated  while  the  remainder  is  under-heated; 
and  while  the  outlet  may  show  a  suitable  temperature,  still 
it  does  not  follow  that  all  parts  of  the  milk  have  been 
thoroughly  treated. 

The  following  simple  method  enables  the  factory  operator 
to  test  the  period  of  exposure  in  the  machine:  Start  the 
machine  full  of  water,  and  after  the  same  has  become  heated 
to  the  proper  temperature,  change  the  inflow  to  full-cream 
milk,  continuing  at  the  same  rate.  Note  the  exact  time  of 
change  and  also  when  first  evidence  .of  milkiness  begins  to 
appear  at  outflow.  If  samples  are  taken  from  first  appear- 
ance of  milky  condition  and  thereafter  at  definite  intervals 
for  several  minutes,  it  is  possible,  by  determining  the  amount 
of  butter-fat  in  the  same,  to  calculate  with  exactness  how 
long  it  takes  for  the  milk  to  entirely  replace  the  water. 

Intermittent  pasteurizers.  Inasmuch  as  the  biological 
and  physical  requirements  of  successful  pasteurizing  neces- 
sitate milk  being  heated  between  the  temperatures  of 
140°  and  160°  F.,  it  is  desirable  that  the  temperature  should 
also  be  under  complete  control.  Moreover,  the  treatment 
should  also  be  in  such  a  way  as  absolutely  to  insure  all  the 
milk  being  treated  for  a  given  period  of  time.  A  fulfill- 
9 


130  Dairy  Bacteriology. 

ment  of  these  conditions  necessitates  the  use  of  the  inter- 
mittent type  of  apparatus,  or  continuous  apparatus  ar- 
ranged so  as  practically  to  conform  to  the  discontinuous 
process. 

The  simplest  way  in  which  these  conditions  can  he  car- 
ried out  is  to  employ  a  number  of  shot-gun  cans  immersed 
in  a  tank  of  hot  water.  By  means  of  this  crude  device,  milk 
or  cream  can  he  pasteurized  more  effectually  than  in  many 
of  the  specially  designed  apparatus.  Tanks  surrounded 
with  water  spaces  can  also  be  used  quite  successfully. 

The  use  of  the  Boyd  cream  ripening  vat  has  been  sug- 
gested, and  this  fulfills  the  necessary  conditions  as'to  a  com- 
mercial pasteurizer.  The  cream  in  this  is  heated  by  means 
of  a  swinging  coil  immersed  in  the  same,  through  which 
hot  water  circulates. 

In  some  of  the  pasteurizers,  steam  is  introduced  directly 
into  the  milk  or  cream,  as  in  Bentley's  apparatus.  It  is 
obvious  that  while  this  may  be  a  cheaper  method  by  which 
to  heat  the  milk,  still  the  proteids  of  the  fluid  must  be 
scalded  in  part,  although  the  temperature  of  the  whole 
mass  may  not  exceed  the  proper  pasteurizing  point.  The 
impurities  in  the  condensed  steam  are  also  objectionable. 

The  first  American  pasteurizer  to  be  built  on  the  intermit- 
tent plan  that  was  made  to  conform  to  biological  require- 
ments was  devised  by  the  writer1  in  1894.  It  consists  of  a 
long,  narrow  vat,  surrounded  by  a  water  chamber  which  is 
heated  by  steam.  To  facilitate  the  heating  of  the  milk, 
both  the  milk  and  water  reservoirs  are  supplied  with  agita- 
tors having  a  to-and-fro  movement. 

The  Potts  pasteurizer  is  another  machine  of  the  inter- 
mittent type  that  has  since  been  quite  generally  introduced 

>  Russell,  Wis.  Expt.  Stat.,  Bull.  44. 


Preservation  of  Milk. 


131 


FIG.  27.    Potts  pasteurizer. 


and  which  conforms  to  the  necessary  biological  conditions. 
This  apparatus  has  a  central  milk  chamber  that  is  sur- 
rounded with  an 
outer  shell  con- 
taining hot  water. 
The  whole  ma- 
chine revolves  on 
a  horizontal  axis, 
and  the  cream  or 
milk  is  thus  thor- 
ough^ agitated 
during  the  heating 
process. 

Coolers,  A  speedy  cooling  of  the  heated  product  is 
essential  to  success  in  pasteurizing.  Some  of  the  machines 
have  been  devised  for  a  combination  purpose,  being  used 
for  the  heating  and  subsequent  cooling  of  the  milk.  This 
is  an  evident  advantage  in  some  ways,  as  it  lessens  the 
amount  of  apparatus  necessary,  also  the  work  involved  in 
cleaning  the  same,  but  at  the  same  time  the  problems  of 
quick  heating  and  cooling  involve  somewhat  different 
principles,  so  that  for  the  most  economical  manipulation  of 
the  product,  separate  pieces  of  apparatus  are  advisable 
where  the  business  warrants  such  expense. 

The  simplest  method  of  treatment  in  cooling  is  to  draw 
off  the  milk  in  shot-gun  cans  and  place  these  first  in  water, 
then  in  ice-water. 

To  cool  milk  most  economically,  two  coolers  should  be 
provided.  With  one  of  these,  cold  water  can  be  used,  and 
with  this  the  temperature  can  be  reduced  to  nearly  that 
of  the  water  in  a  short  time.  In  order,  however,  to 
lower  the  temperature  below  a  point  where  spore  germi- 
nation will  readily  occur,  milk  should  be  chilled  by  the 


132  Dairy  Bacteriology. 

aid  of  ice.  This  may  be  applied  in  the  same  cooler  that  is 
used  for  running  cold  water,  by  supplying  ice-water  for  the 
latter  part  of  the  cooling  process.  To  use  ice  economically, 
the  ice  itself  should  be  applied  as  closely  as  possible  to  the 
milk  to  be  cooled,  for  the  larger  part  of  the  chilling  value 
of  ice  comes  from  the  melting  of  the  same.  To  convert  a 
pound  of  ice  at  32°  F.  into  a  pound  of  water  at  the  same 
temperature,  if  we  disregard  radiation,  would  require  as 
much  heat  as  would  suffice  to  raise  142  pounds  of  water 
one  degree  F.,  or  one  pound  of  water  142°  F.  The  ab- 
sorptive capacity  of  milk  for  heat  (specific  heat)  is  not  quite 
the  same  as  it  is  with  water,  being  .847  for  milk  in  com- 
parison with  1.0  for  water.1  Hot  milk  would  therefore 
require  somewhat  less  ice  to  cool  it  than  would  be  required 
by  an  equal  volume  of  water  at  the  same  temperature. 

In  the  mere  melting  of  a  pound  of  ice,  if  expended  on 
cooling  heated  milk,  the  temperature  of  pasteurized  milk 
would  be  reduced  to  a  keeping  temperature.  To  take  ad- 
vantage of  this,  the  ice  should  be  brought  in  close  contact 
with  the  milk,  rather  than  to  utilize  the  specific  heat  in 
cooling  water  which  is  later  applied  to  the  milk.  If  broken 
ice  is  used  directly,  it  is  utilized  most  economically  if  the 
milk  surrounds  it,  as  in  this  way  the  ice  does  not  absorb 
heat  from  the  outside. 

Bacterial  efficiency  of  pasteurizing;  apparatus.  The  bac- 
terial content  of  pasteurized  milk  and  cream  will  depend 
somewhat  on  the  number  of  organisms  originally  present 
in  the  same.  Naturally,  if  mixed  milk  brought  to  a  cream- 
ery is  pasteurized,  the  number  of  organisms  remaining 
after  treatment  would  be  greater  than  if  the  raw  material 
was  fresh  and  produced  on  a  single  farm. 

An  examination  of  milk  and  cream  pasteurized  on  a  com- 

1  Fleischmann,  Landw.  Versuchsstat.,  17: 251. 


Preservation  of  Milk. 


133 


mercial  scale  in  the  Russell  vat  at  the  Wisconsin  Dairy 
school  showed  that  over  99.8  per  cent  of  the  bacterial  life  in 

raw  milk  or  cream 
was  destroyed  by 
the  heat  employed, 
i.  e.,  155°  F.  for 
twenty  minutes  du- 
ration.1 In  nearly 
one-half  of  the  sam- 
ples of  milk,  the 
germ  content  in  the 
pasteurized  sample 
fell  below  1,000 
bacteria  per  cc., 
and  the  average  of 
twenty-five  samples 
contained  6,140  bac- 

FIG.  28.    Effect  of  pasteurizing  on  germ  content  £erja     per     cc         Jn 
of  milk.    Black  square  represents  bacteria  of  raw 
milk;    small  white  square,  those  remaining  after  Cream  the  germ  COn- 

pasteurization.  tent  was  higher,  av- 

eraging about  25,000  bacteria  per  cc.  This  milk  was  taken 
from  the  general  creamery  supply,  which  was  high  in  organ- 
isms, containing  on  an  average  3,675,000  bacteria  per  cc. 
De  Schweinitz2  has  reported  the  germ  content  of  a  supply 
furnished  in  Washington  which  was  treated  at  158°  to  160° 
F.  for  fifteen  minutes.  This  supply  came  from  a  single 
source.  Figures  reported  were  from  48-hour-old  agar  plates. 
Undoubtedly  these  would  have  been  higher  if  a  longer  pe- 
riod of  incubation  had  been  maintained.  The  average  of 
82  samples,  taken  for  the  period  of  one  year,  showed  325 
bacteria  per  cc. 


1  Russell,  12  Wis.  Expt.  Stat.  Kept ,  1895,  p.  160. 
2De  Schweinitz,  Nat.  Med.  Rev.,  1899,  No.  11. 


CHAPTER  VII. 
BACTERIA  AND  BUTTER-MAKING. 

IN  making  butter  from  the  butter  fat  in  milk,  it  is  neces- 
sary to  concentrate  the  fat  globules  into  cream,  preliminary 
to  the  churning  process.  The  cream  may  be  raised  by  the 
gravity  process  or  separated  from  the  milk  by  centrifugal 
action.  In  either  case  the  bacteria  that  are  normally  pres- 
ent in  the  milk  differentiate  themselves  in  varying  numbers 
in  the  cream  and  the  skim-milk.  The  cream  always  con- 
tains per  cc.  a  great  many  more  than  the  skim-milk,  the 
reason  for  this  being  that  the  bacteria  are  caught  and  held 
in  the  masses  of  fat  globules,  which,  on  account  of  their 
lighter  specific  gravity,  move  toward  the  surface  of  the 
milk  or  toward  the  interior  of  the  separator  bowl.  This 
filtering  action  of  the  fat  globules  is  similar  to  what  happens 
in  muddy  water  upon  standing.  As  the  suspended  particles 
fall  to  the  bottom  they  carry  with  them  a  large  number  of 
the  organisms  that  are  in  the  liquid. 

Various  creaming  methods.  The  creaming  method  has 
an  important  bearing  on  the  kind  as  well  as  the  number 
of  the  bacteria  that  are  to  be  found  in  the  cream.  The 
difference  in  species  is  largely  determined  by  the  difference 
in  ripening  temperature,  while  the  varying  number  is  gov- 
erned more  by  the  age  of  the  milk. 

1.  Primitive  gravity  methods.  In  the  old  shallow-pan 
process,  the  temperature  of  the  milk  is  relatively  high,  as 
the  milk  is  allowed  to  cool  naturally.  This  comparatively 
high  temperature  favors  especially  the  development  of 
those  forms  whose  optimum  growing-point  is  near  the  air 


Bacteria  and  Butter- Making.  135 

temperature.  By  this  method  the  cream  layer  is  exposed 
to  the  air  for  a  longer  time  than  with  any  other,  and  conse- 
quently the  contamination  from  this  source  is  greater. 
Usually  cream  obtained  by  the  shallow-pan  process  will 
contain  a  larger  number  of  species  and  also  have  a  higher 
acid  content. 

2.  Modern  gravity  methods.     In  the  Cooley  process,  or 
any  of  the  modern  gravity  methods  where  cold  water  or  ice 
is  used  to  lower  the  temperature,  the  conditions  do  not 
favor  the  growth  of  a  large  variety  of  species.    The  number 
of  bacteria  in  the  cream  will  depend  largely  upon  the  man- 
ner in  which  the  milk  is  handled  previous  to  setting.     If 
care  is  used  in  milking,  and  the  milk  is  kept  so  as  to  ex- 
clude outside  contamination,  the  cream  will  be  freer  from 
bacteria  than  if  carelessness  prevails  in  handling  the  milk. 
Only  those  forms  will  develop  in  abundance  that  are  able 
to  grow  at  the  low  temperature  at  which  the  milk  is  set. 
Cream  raised  by  this  method  is  less  frequently  infected  with 
undesirable  forms  than  that  which  is  creamed  at  a  higher 
temperature. 

3.  Centrifugal  method.     Separator  cream  should  contain 
less  germ-life  than  that  which  is  secured  in  the  old  way. 
It  should  contain  only  those  forms  that  have  found  their 
way  into  the  milk  during  and  subsequent  to  the  milking, 
for  the  cream  is  ordinarily  separated  so  soon  that  there  is 
but  little  opportunity  of  infection,  if  care  is  taken  in  the 
handling.     As  a  consequence,  the  number  of  species  found 
therein  is  smaller. 

Where  milk  is  separated,  it  is  always  prudent  to  cool  the 
cream  so  as  to  check  growth,  as  the  milk  is  generally  heated 
before  separating  in  order  to  skim  efficiently. 

Although  cream  is  numerically  much  richer  in  bacteria 


138  Dairy  Bacteriology. 

than  milk,  yet  the  changes  due  to  bacterial  action  are  slower 
so  that  it  usually  spoils  sooner  than  cream.  For  this  same 
reason,  cream  will  sour  sooner  when  it  remains  on  the  milk 
than  it  will  if  it  is  separated  as  soon  as  possible.  This  fact 
indicates  the  necessity  of  early  creaming,  so  as  to  increase 
the  keeping  quality  of  the  product,  and  is  another  argu- 
ment in  favor  of  the  separator  process. 

Ripening  Of  cream.  If  cream  is  allowed  to  remain  at 
ordinary  temperatures,  it  undergoes  a  series  of  fermenta- 
tion changes  that  are  exceedingly  complex  in  character,  the 
result  of  which  is  to  produce  in  butter  made  from  the 
same  the  characteristic  flavor  and  aroma  that  are  so  well 
known  in  this  article.  We  are  so  accustomed  to  the  de- 
velopment of  these  flavors  in  butter  that  they  are  not  gen- 
erally recognized  as  being  intimately  associated  with  bac- 
terial activity  unless  compared  with  butter  made  from  per- 
fectly fresh  cream.  Sweet-cream  butter  lacks  the  aromatic 
principle  that  is  prominent  in  the  ripened  product,  and 
while  the  flavor  is  delicate,  it  is  relatively  unpronounced. 

In  the  primitive  method  of  butter-making,  where  the  but- 
ter was  made  on  the  farm,  the  ripening  of  cream  became  a 
necessity  in  order  that  sufficient  material  might  be  accumu- 
lated to  make  a  churning.  The  ripening  change  occurred 
spontaneously  without  the  exercise  of  any  especial  control. 
With  the  development  of  the  creamery  system  came  the 
necessity  of  exercising  a  control  of  this  process,  and  there- 
fore the  modern  butter-maker  must  understand  the  prin- 
ciples which  are  involved  in  this  series  of  complex  changes 
that  largely  give  to  his  product  its  commercial  value. 

In  these  ripening  changes  three  different  factors  are  to 
be  taken  into  consideration:  the  development  of  acid,  flavor 
and  aroma.  Much  confusion  in  the  past  has  arisen  from  a 


Bacteria  and  Butter- Making.  137 

failure  to  discriminate  between  these  qualities.  While  all 
three  are  produced  simultaneously  in  ordinary  ripening,  it 
does  not  necessarily  follow  that  they  are  produced  by  the 
same  cause.  If  the  ripening  changes  are  allowed  to  go  too 
far,  undesirable  rather  than  beneficial  decomposition  pro- 
ducts are  produced.  These  greatly  impair  the  value  of  but- 
ter, so  that  it  becomes  necessary  to  know  just  to  what  extent 
this  process  should  be  carried. 

In  cream  ripening  there  is  a  very  marked  bacterial 
growth,  the  extent  of  which  is  determined  mainly  by  the 
temperature  of  the  cream.  Conn  and  Esten  l  find  that  the 
number  of  organisms  may  vary  widely  in  unripened  cream, 
but  that  the  germ  content  of  the  ripened  product  is  more 
uniform.  When  cream  is  ready  for  the  churn,  it  often 
contains  500,000,000  organisms  per  cc.,  and  frequently 
even  a  higher  number.  This  represents  a  germ  content 
that  has  no  parallel  in  any  natural  material. 

The  larger  proportion  of  bacteria  in  cream  as  it  is  found 
in  the  creamery  belong  to  the  acid-producing  class,  but  in 
the  process  of  ripening,  these  forms  seem  to  thrive  still 
better,  so  that  when  it  is  ready  for  churning  the  germ  con- 
tent of  the  cream  is  practically  made  up  of  this  type. 

Effect  on  Churning.  In  fresh  cream  the  fat  globules  which 
are  suspended  in  the  milk  serum  are  surrounded  by  a  film  of 
albuminous  material  which  prevents  them  from  coalescing 
readily.  During  the  ripening  changes,  this  enveloping 
substance  is  modified,  probably  by  partial  solution,  so  that 
the  globules  cohere  when  agitated,  as  in  churning.  The 
result  is  that  ripened  cream  churns  more  easily,  and  as  it 
is  possible  to  cause  a  larger  number  of  the  smaller  fat- 
globules  to  cohere  to  the  butter  granules,  the  yield  is 

1  Conn  and  Esten,  Cent.  f.  Bakt.,  II  Abt.,  1901,  7:  746. 


138  Dairy  Bacteriology. 

slightly  larger — a  point  of  considerable  economic  impor- 
tance where  large  quantities  of  butter  are  made. 

Development  Of  acid.  The  result  of  this  enormous  bac- 
terial multiplication  is  that  acid  is  produced  in  cream,  lac- 
tic being  the  principal  acid  so  formed. 

Other  organic  acids  are  undoubtedly  formed  as  well  as 
certain  aromatic  products.  While  the  production  of  acid 
as  a  result  of  fermentative  activity  is  usually  accompanied 
with  a  development  of  flavor,  the  flavor  is  not  directly  pro- 
duced by  the  formation  of  acid.  If  cream  is  treated  in 
proper  proportions  with  a  commercial  acid,  as  hydrochloric,1 
it  assumes  the  same  churning  properties  as  found  in  nor- 
mally ripened  cream,  but  is  devoid  of  the  desire J  aromatic 
qualities.  Lactic  acid8  has  also  been  used  in  a  similar  way 
but  with  no  better  results. 

The  amount  of  acidity  that  should  be  developed  under 
natural  conditions  so  as  to  secure  the  optimum  quality  a» 
to  flavor  and  aroma  is  the  most  important  question  in 
cream  ripening.  Concerning  this  there  have  been  two  some- 
what divergent  views  as  to  what  is  best  in  practice,  some 
holding  that  better  results  were  obtained  with  cream  rip- 
ened to  a  high  degree  of  acidity  than  where  a  less  amount 
was  developed.3  The  present  tendency  seems  to  be  to  de- 
velop somewhat  more  than  formerly,  as  it  is  thought  that 
this  secures  more  of  the  "high,  quick"  flavor  wanted  in 
the  market.  On  the  average,  cream  is  ripened  to  about 
0.5  to  0.65  per  cent  acidity,  a  higher  percentage  than  this 
giving  a  strong-flavored  butter.  In  the  determination  of 
acidity,  the  most  convenient  method  is  to  employ  the  Far- 

'Tiemann,  Milch  Zeit.,  23:701. 

•  Milch  Zeit,  1889,  p.  7;  1894,  p.  634:  1895,  p.  383. 

«  Dean,  Out.  Agr.  Coll.,  1897,  p.  60. 


Bacteria  and  Butter- Making.  139 

rington  alkaline  tablet,  which  permits  of  an  accurate  and 
rapid  estimation  of  the  acidity  in  the  ripening  cream.  The 
amount  of  acidity  to  be  produced  must  of  necessity  be  gov- 
erned by  the  amount  of  butter-fat  present,  for  the  forma- 
tion of  acid  is  confined  to  the  serum  of  the  cream;  conse- 
quently, a  rich  cream  would  show  less  acid  by  titration 
than  a  thinner  cream,  and  still  contain  really  as  much  acid 
as  the  other.  The  importance  of  this  factor  is  evident  in 
gathered-cream  factories. 

The  rate  of  ripening  is  dependent  upon  the  conditions 
that  affect  the  rate  of  growth  of  bacterial  life,  such  as  time 
and  temperature,  number  of  organisms  in  cream  and  also 
the  per  cent  of  butter  fat  in  the  cream.  Some  years  ago 
it  was  customary  to  ripen  cream  at  about  50°  to  60°  F., 
but  more  recently  better  results  have  been  obtained,  it  is 
claimed,  where  the  ripening  temperature  is  increased  and 
the  period  of  ripening  lessened.  As  high  a  temperature 
as  TO0  to  75°  F.  has  been  recommended.  It  should  be  said 
that  this  variation  in  practice  may  have  a  valid  scientific 
foundation,  for  the  temperature  of  the  ripening  cream  is  un- 
doubtedly the  most  potent  factor  in  determining  what 
kind  of  bacteria  will  develop  most  luxuriantly.  It  is  well 
known  that  those  forms  that  are  capable  of  producing  bit- 
ter flavors  are  able  to  thrive  better  at  a  lower  temperature 
than  some  of  the  desirable  ripening  species. 

The  importance  of  this  factor  would  be  lessened  where 
a  pure  culture  was  used  in  pasteurized  cream,  because  here 
practically  the  selected  organism  alone  controls  the  field. 

It  is  frequently  asserted  that  better  results  are  obtained 
by  stirring  the  cream  and  so  exposing  it  to  the  air  as  much 
as  possible.  Experiments  made  at  the  Ontario  Agricul- 
tural College,  however,  show  practically  no  difference  in 


140  Dairy  Bacteriology. 

the  quality  of  the  butter  made  by  these  two  methods. 
The  great  majority  of  the  bacteria  in  the  cream  belong  to 
the  facultative  class,  and  are  able  to  grow  under  conditions 
where  they  are  not  in  direct  contact  with  the  air, 

Flavor  and  aroma.  The  basis  for  the  peculiar  flavor  or 
taste  which  ripened  cream-butter  possesses  is  due,  in  large 
part,  to  the  formation  of  certain  decomposition  products 
formed  by  various  bacteria.  Aroma  is  a  quality  often 
confounded  with  flavor,  but  this  is  produced  by  volatile 
products  only,  which  appeal  to  the  sense  of  smell  rather 
than  taste.  Generally  a  good  flavor  is  accompanied  by  a 
desirable  aroma,  but  the  origin  of  the  two  qualities  is 
not  necessarily  dependent  on  the  same  organisms.  The 
quality  of  flavor  and  aroma  in  butter  is,  of  course,  also  af- 
fected by  other  conditions,  as,  for  instance,  the  presence  or 
absence  of  salt,  as  well  as  the  inherent  qualities  of  the 
milk,  that  are  controlled,  to  some  extent  at  least,  by  the 
character  of  the  feed  which  is  consumed  by  the  animal. 
The  exact  source  of  these  desirable  butevancescent  qualities 
in  butter  is  not  yet  satisfactorily  determined.  According 
to  Storch,1  flavors  are  produced  by  the  decomposition  of 
the  milk  sugar  and  the  absorption  of  the  volatile  flavors 
by  the  butter  fat.  Conn  2  holds  that  the  nitrogenous  ele- 
ments in  cream  serve  as  food  for  bacteria,  and  in  the  de- 
composition of  which  the  desired  aromatic  substance  is  pro- 
duced. The  change  is  unquestionably  a  complex  one,  and 
cannot  be  explained  as  a  single  fermentation. 

There  is  no  longer  much  doubt  but  that  both  acid-forming 
and  casein-digesting  species  can  take  part  in  the  production 
of  proper  flavors  as  well  as  desirable  aromas.  The  researches 

1  Storch,  Nogle.  Unders.  over  Floed.  Syrning,  1890. 

2  Conn,  6  Storrs  Expt.  Stat.,  1893,  p.  66. 


Bacteria  and  Butter-Making.  141 

of  Conn,1  who  lias  studied  this  question  most  exhaustively, 
indicate  that  both  of  these  types  of  decomposition  partici- 
pate in  the  production  of  flavor  and  aroma.  He  has  shown 
that  both  flavor  and  aroma  production  are  independent 
of  acid;  that  many  good  flavor-producing  forms  belong 
to  that  class  which  renders  milk  alkaline,  or  does  not 
change  the  reaction  at  all.  Some  of  these  species  liquefied 
gelatin  and  would  therefore  belong  to  the  casein-dissolv- 
ing class.  Those  species  that  produced  bad  flavors  are  also 
included  in  both  fermentative  types.  Conn  has  found 
a  number  of  organisms  that  are  favorable  flavor-pro- 
ducers; in  fact  they  were  much  more  numerous  than  de- 
sirable aroma-yielding  species.  None  of  the  favorable 
aroma  forms  according  to  his  investigations  were  lactic- 
acid  species, —  a  view  which  is  also  shared  by  Weigmann.9 

McDonnell8  has  found  that  the  production  of  aroma  in 
certain  cases  varies  at  different  temperatures,  the  most  pro- 
nounced being  evolved  near  the  optimum  growing  tem- 
perature, which,  as  a  general  rule,  is  too  high  for  cream 
ripening. 

The  majority  of  bacteria  in  ripening  cream  do  not  seem 
to  exert  any  marked  influence  in  butter.  A  considerable 
number  of  species  are  positively  beneficial,  inasmuch  as 
they  produce  a  good  flavor  or  aroma.  A  more  limited 
number  are  concerned  in  the  production  of  undesirable 
ripening  changes.  This  condition  being  true,  it  may  seem 
strange  that  butter  is  as  good  as  it  is,  because  so  frequently 
the  requisite  care  is  not  given  to  the  development  of  proper 
ripening.  In  all  probability  the  chief  reason  why  this  is 
so  is  that  those  bacteria  that  find  milk  and  cream  pre-emi- 

i  Conn,  9  Storrs  Expt.  Stat.,  1896,  p.  17. 

a  Wiegmann,  Milch  Zeit.,  1891,  p.  793. 

•  McDonnell,  u.  Milchsaure  Bakterien  (Diss.  Kiel,  1899),  p.  43. 


142  Dairy  Bacteriology. 

nently  suited  to  their  development,  e.  g.  the  lactic-acid 
class,  are  either  neutral  or  beneficial  in  their  effect  on 
butter. 

Use  of  Starters.  Experience  has  amply  demonstrated 
that  it  is  possible  to  control  the  nature  of  the  fermentative 
changes  that  occur  in  ripening  cream  to  such  an  extent  as 
to  materially  improve  the  quality  of  the  butter.  This  is 
frequently  done  by  the  addition  of  a  "  starter."  While 
starters  have  been  employed  for  man}' years  for  the  purpose 
mentioned,  it  is  only  recently  that  their  nature  has  been 
understood.  A  starter  may  be  selected  from  widely  diver- 
gent sources,  but  in  all  cases  it  is  sure  to  contain  a  large 
number  of  bacteria,  and  the  presumption  is  that  they  are 
of  such  a  nature  as  to  produce  desirable  fermentative 
changes  in  the  cream. 

In  the  selection  of  these  so-called  natural  starters,  it  fol- 
lows that  they  must  be  chosen  under  such  conditions  as 
experience  has  shown  to  give  favorable  results.  For  this 
purpose,  whole  milk  from  a  single  animal  is  often  used 
where  the  same  is  observed  to  sour  with  the  production  of 
no  gas  or  other  undesirable  taint.  A  skim-milk  starter 
from  a  mixed  supply  is  recommended  by  many.  Butter 
milk  is  frequently  employed,  but  in  the  opinion  of  butter 
experts  is  not  as  suitable  as  the  others  mentioned. 

It  not  infrequently  happens  that  the  practical  operator 
may  be  misled  in  selecting  a  starter  that  is  not  desirable, 
or  by  continuing  its  use  after  it  has  become  contaminated. 

In  1890 1  a  new  system  of  cream  ripening  was  intro- 
duced in  Denmark  by  Storch  that  possesses  the  merit  of 
being  a  truly  scientific  and  at  the  same  time  practical 
method.  This  consisted  in  the  use  of  pure  cultures  of 

» Storch,  Milch  Zeit.,  1890,  p.  304. 


Bacteria  and  Butter-Making,  143 

specific  organisms  that  were  selected  on  account  of  their 
ability  to  produce  a  desirable  ripening  change  in  cream. 
The  introduction  of  these  so-called  culture  starters  has  be- 
come almost  universal  in  Denmark  and  in  parts  of  Ger- 
many. Their  use  is  also  extending  in  this  country,  Aus- 
tralia and  New  Zealand. 

Principles  of  pure-culture  cream-ripening:.  In  the  proper 
use  of  pure  cultures  for  ripening  cream,  it  is  necessary  first 
to  eliminate  as  far  as  possible  the  bacteria  already  present 
in  cream  before  the  culture  starter  is  added.  This  result  is 
accomplished  by  heating  the  cream  to  a  temperature  suf- 
ficiently high  to  destroy  the  vegetating  organisms.  The 
addition  of  a  properly  selected  starter  will  then  give  the 
chosen  organism  such  an  impetus  as  will  generally  enable 
it  to  gain  the  ascendency  over  any  other  bacteria  and  so 
control  the  character  of  the  ripening.  The  principle  em- 
ployed is  quite  like  that  practiced  in  raising  grain.  The 
farmer  prepares  his  soil  by  plowing,  in  this  way  killing  the 
weeds.  Then  he  sows  his  selected  grain,  which  is  merely  a 
pure  culture,  and  by  the  rapid  growth  of  this,  other  forms 
are  held  in  check. 

The  attempt  has  been  made  to  use  these  culture  starters 
in  raw  sweet  cream,  but  it  can  scarcely  be  expected  that 
the  most  beneficial  results  will  be  attained  in  this  way. 
This  method  has  been  justified  on  the  basis  of  the  following 
experiments.  Where  cream  is  pasteurized  and  no  starter 
is  added,  the  spore-bearing  forms  frequently  produce  unde- 
sirable flavors.  These  can  almost  always  be  controlled  if 
a  culture  starter  is  added,  the  obnoxious  form  being  re- 
pressed by  the  presence  of  the  added  starter.  This  condi- 
tion is  interpreted  as  indicating  that  the  addition  of  a  starter 
to  cream  which  already  contains  developing  bacteria  will 


Dairy  Bacteriology. 

prevent  those  originally  present  in  the  cream  from  grow- 
ing.1 This  repressive  action  of  one  species  on  another  is  a 
well-known  bacteriological  fact,  but  it  must  be  remembered 
that  such  an  explanation  is  only  applicable  in  those  cases 
where  the  culture  organism  is  better  able  to  develop  than 
those  forms  that  already  exist  in  the  cream. 

If  the  culture  organism  is  added  to  raw  milk  or  cream 
which  already  contains  a  flora  that  is  well  suited  to  develop 
in  this  medium,  it  is  quite  doubtful  whether  it  would  gain 
the  supremacy  in  the  ripening  cream.  The  above  method 
of  adding  a  culture  to  raw  cream  renders  cream-ripening 
details  less  burdensome,  but  at  the  same  time  Danish  ex- 
perience, which  is  entitled  to  most  credence  on  this  ques- 
tion, is  opposed  to  this  method. 

Reputed  advantages  of  culture  starters.  1.  Flavor  and 
aroma.  Naturally  the  flavor  produced  by  pure-culture  fer- 
ments depends  upon  the  character  of  the  organism  used. 
Those  which  are  most  extensively  used  are  able  to  produce 
a  perfectly  clean  but  mild  flavor,  and  a  delicate  but  not 
pronounced  aroma.  The  uhigh,  quick"  flavor  and  aroma 
that  is  so  much  desired  in  the  American  market  is  not 
readily  obtained  by  the  use  of  cultures.  It  is  quite  problem- 
atical whether  the  use  of  any  single  species  will  give  any 
more  marked  aroma  than  normally  occurs  in  natural  ripen- 
ing. 

2.  Uniformity  of  product.  Culture  starters  produce  a 
more  uniform  product  because  the  type  of  fermentation  is 
under  more  complete  control,  and  herein  is  the  greatest 
advantage  to  be  derived  from  their  use.  Even  the  best 
butter-maker  at  times  will  fail  to  secure  uniform  results 
if  his  starter  is  not  perfectly  satisfactory. 

1  Conn,  9  Storrs  Expt.  Stat,  1890,  p.  25. 


Bacteria  and  Butter-Making.  145 

3.  Keeping  quality  of  product.     Butter  made  from  pas- 
teurized cream  to  which  a  pure-culture  starter  has  been 
added  will  keep  much  better  than  the  ordinary  product,  be- 
cause the  diversity  of  the  bacterial  flora  is  less  and  the  milk 
is  therefore  not  so  likely  to  contain  those  organisms  that 
produce  an  "off"  condition. 

4.  Elimination  of  taints.     Many  defective  conditions  in 
butter  are  attributable  to  the  growth  of  undesirable  bacteria 
in  the  cream  that  result  in  the  formation  of  "off"  flavors 
and  taints.  If  cream  is  pasteurized,  thereby  destroying  these 
organisms,  then  ripened  with  pure  ferments,  it  is  generally 
possible  to   eliminate  the   abnormal  conditions.1     Taints 
may  also  be  present  in  cream  due  to  direct  absorption  from 
the  cow  or  through  exposure  to  foul  odors.2     Troubles  of 
this  sort  may  thus  be  carried  over  to  the  butter.     This  is 
particularly  true  in  regions  where  leeks  and  wild  onions 
abound,  as  in  some  of  the  Atlantic  States.     The  heating  of 
the  cream  tends  to  expel  these  volatile  taints,  so  that  a 
fairly  good  article  of  butter  can  be  made  from  what  would 
otherwise  be  a  relatively  worthless  product. 

Characteristics  desired  in  culture  starters.  Certain  con- 
ditions as  the  following  are  desirable  in  starters  made  from 
pure  cultures: 

1.  Vigorous  growth  in  milk  at  ordinary  ripening  tem- 
peratures. 

2.  Ability  to  form  acid  so  as  to  facilitate  churning  and 
increase  the  yield  of  butter. 

3.  Able  to  produce  a  clean  flavor  and  desirable  aroma. 

4.  Impart  a  good  keeping  quality  to  butter. 

i  Milch  Zeit.,  1891,  p.  122;  1894,  p.  284;  1895,  p.  56;  1896,  p.  163. 
8  McKay,  Bull.  32,  Iowa  Expt.  Stat,  p.  477. 
10 


146  Dairy  Bacteriology. 

5.  Not  easily  modified  in  its  flavor-producing  qualities 
by  artificial  cultivation. 

These  different  conditions  are  difficult  to  attain,  for  the 
reason  that  some  of  them  seem  to  be  in  part  incompatible. 
Weigmann1  found  that  a  good  aroma  was  generally  an 
evanescent  property,  and  therefore  opposed  to  good  keep- 
ing quality.  Conn  has  shown  that  the  functions  of  acid- 
formation,  flavor  and  aroma  production  are  not  necessarily 
related,  and  therefore  the  .chances  of  finding  a  single  or- 
ganism that  possesses  all  the  desirable  attributes  are  not 
very  good. 

In  all  probability  no  one  germ  possesses  all  of  these  de- 
sirable qualities,  but  natural  ripening  is  the  resultant  of 
the  action,  of  several  forms.8  This  idea  has  led  to  the  at- 
tempt at  mixing  selected  organisms  that  have  been  chosen 
on  account  of  certain  favorable  characteristics  which  they 
might  possess.  The  difficulty  of  maintaining  such  a  com- 
posite culture  in  its  correct  proportions  when  it  is  propa- 
gated in  the  creamery  is  seemingly  well  nigh  insuperable, 
as  one  organism  is  very  apt  to  develop  more  or  less  rapidly 
than  the  other. 

A  very  satisfactory  way  in  which  these  cultures  are  mar- 
keted is  to  mix  the  bacterial  growth  with  some  sterile, 
inert,  dry  substance.  This  is  the  method  used  in  most  of 
the  Danish  cultures.  In  this  country,  some  of  the  more 
prominent  cultures  employed  are  marketed  in  a  liquid  form. 

Culture  vs.  home-made  starters.  One  great  advantage 
which  has  accrued  from  the  use  of  culture  or  commercial 
starters  has  been  that  in  emphasizing  the  need  of  closer 
control  of  the  ripening  process,  greater  attention  has  been 

»  Weigmann,  Lamlw.  Woch.  f.  Schl.  Hoi.,  No.  2,  1890. 
«  Weigmann,  Cent.  f.  Bakt.,  II  Abt.,  3:49  7,  1897. 


Bacteria  and  Butter- Making.  147 

paid  to  the  carrying  out  of  the  details.  In  the  hands  of 
the  better  operators,  the  differences  in  flavor  of  butter 
made  with  a  culture  or  a  natural  starter  are  not  marked,1 
but  in  the  hands  of  those  who  fail  to  make  a  good  product 
under  ordinary  conditions,  an  improvement  is  often  se- 
cured where  a  commercial  culture  is  used. 

Pasteurization  as  applied  to  butter-making:.  This  pro- 
cess, as  applied  to  butter  making,  is  often  confounded  with 
the  treatment  of  milk  and  cream  for  direct  consumption. 
It  is  unfortunate  that  the  same  term  is  used  in  connection 
with  the  two  methods,  for  they  have  but  little  in  common 
except  in  the  use  of  heat  to  destroy  the  germ  life  of  the 
milk.  In  pasteurizing  cream  for  butter-making,  it  is  not 
•necessary  to  observe  the  stringent  precautions  that  are  to 
be  noted  in  the  preservation  of  milk;  for  the  addition  of  a 
rapidly  developing  starter  controls  at  once  the  fermenta- 
tive changes  that  subsequently  occur.  Then  again,  the 
physical  requirement  as  to  the  production  of  a  cooked  taste 
is  not  so  stringent  in  butter-making.  While  a  cooked 
taste  is  imparted  to  milk  or  even  cream  at  about  158P  F.r 
it  is  possible  to  make  butter  that  shows  no  permanent 
cooked  taste  from  cream  that  has  been  raised  as  high  as 
185°  or  even  195°  F.  This  is  due  to  the  fact  that  the  fat 
does  not  readily  take  up  those  substances  that  give  to 
scalded  milk  its  peculiar  flavor. 

Unless  care  is  taken  in  the  manipulation  of  the  heated 
cream,  the  grain  or  body  of  the  butter  may  be  injured. 
This  tendency  can  be  overcome  if  the  ripened  cream  is 

i  At  the  National  Creamery  Buttermakers1  Association  lor  1901,  193  out  of  240 
exhibitors  used  starters.  Of  those  that  employed  starters,  nearly  one-half  used 
commercial  cultures.  There  was  practically  no  difference  in  the  average  score 
of  the  two  classes  of  starters,  but  those  using  starters  ranked  nearly  two  points 
higher  in  flavor  than  those  that  did  not. 


148  Dairy  Bacteriology. 

chilled  to  48°  F.  for  about  two  hours  before  churning.1  It 
is  also  essential  that  the  heated  cream  should  be  quickly 
and  thoroughly  chilled  after  being  pasteurized. 

The  Danes,  who  were  the  first  to  employ  pasteurization 
in  butter-making,  used,  in  the  beginning,  a  temperature 
ranging  from  158°  to  167°  F.,  but  owing  to  the  prevalence 
of  such  diseases  as  tuberculosis  and  foot-and-mouth  disease, 
it  became  necessary  to  treat  the  milk  so  as  to  thoroughly 
destroy  the  virus  of  the  disease.  This  can  be  done  by  mo- 
mentarily heating  the  same  to  the  temperature  of  185°  F., 
and  this  temperature  is  now  generally  employed.  With 
the  use  of  this  higher  temperature  the  capacity  of  the  pas- 
teurizing apparatus  is  considerably  reduced,  as  not  more 
than  one-half  to  two-thirds  as  much  milk  can  be  handled 
at  185°  as  at  158°  F. 

When  the  system  was  first  introduced  in  Denmark,  two 
methods  of  procedure  were  followed:  the  whole  milk  was 
either  heated  before  separation,  or  the  cream  was  pasteur- 
ized afterwards,  the  skim  milk  being  treated  separately. 
At  the  present  time  the  latter  system  is  gaining  grou'nd. 

The  present  law  makes  it  compulsory  to  heat  all  skim 
milk  to  185°  F.  to  avoid  the  dissemination  of  the  diseases 
previously  mentioned. 

Apparatus  for  pasteurizing.  As  it  is  not  necessary  to 
heat  the  milk  or  cream  for  butter-making  under  such  a 
narrow  range  of  conditions  as  when  designed  for  direct  con- 
sumption, it  is  permissible  to  employ  machinery  that  be- 
longs to  the  continuous-flow  type.  These  pasteurizers  have 
a  large  capacity  and  it  is  possible  to  handle  in  them  several 
thousand  pounds  per  hour.  The  majority  of  apparatus  for 
this  purpose  has  originated  in  Denmark  and  Germany. 

*  N.  Y.  Prod.  Rev.,  October,  1899. 


Bacteria  and  .Butter- 


A  quantitative  determination  of  the  bacteria  found  in  milk 
and  cream  when,  treated  in  machinery  of  this  class  almost 
always  shows  a  degree  of  variation  in  results  that  is  not  to 
be  noted  in  the  discontinuous  apparatus. 


FIG.  29.    Reid's  Continuous  Pasteurizer. 

Harding  and  Rogers l  have  tested  the  efficiency  of  one 
of  the  Danish  type  of  continuous  pasteurizers.  These  ex- 
periments were  made  at  158°,  176°  and  185°  F.  They 

*  Harding  and  Rogers,  Bull.  182,  N.  Y.  (Geneva)  Expt.  Stat.,  Dec.,  1899. 


150  Dairy  Bacteriology. 

found  the  efficiency  of  the  machine  not  wholly  satisfactory 
at  the  lower  temperatures.  At  158°  F.  the  average  of  four- 
teen tests  gave  15,300  bacteria  per  cc.,  with  a  maximum  to 
minimum  range  from  62,790  to  120.  Twenty-five  examina- 
tions at  176°  F.  showed  an  average  of  only  117,  with  a 
range  from  300  to  20.  The  results  at  185°  F.  showed 
practically  the  same  results  as  noted  at  176°  F.  Consider- 
able trouble  was  experienced  with  the  "  scalding  on  "  of 
the  milk  to  the  walls  of  the  machine  when  milk  of  high 
acidity  was  used. 

The  writer1  tested  Reid's  pasteurizer  at  155°  to  165°  F. 
with  the  following  results:  in  some  cases  as  many  as  40 
per  cent  of  the  bacteria  survived,  which  number  in  some 
cases  exceeded  2,000,000  bacteria  per  cc. 

Development  of  pasteurization  and  pure-culture  ripening. 

Since  the  introduction  of  this  system  into  Denmark  in 
1890,  creamery  methods  have  been  completely  revolution- 
ized. At  the  present  time  practically  all  of  the  butter  ex- 
ported to  England  is  prepared  in  this  way  and  by  far  the 
larger  part  of  that  which  is  consumed  at  home.  There  are 
several  different  selected  commercial  cultures  that  are  used. 
In  Sweden,  in  1897,  67  per  cent  of  creameries  pasteurized 
their  product. 

In  Germany  the  system  has  been  adopted  most  exten- 
sively in  the  north,  and  may  be  said  to  be  practically  an  ex- 
tension of  the  Danish  system.  In  southern  Germany  the 
method  is  not  employed  to  any  extent. 

In  this  country  considerable  agitation  has  been  given 
the  matter,  but  the  process  has  been  but  slowly  adopted. 
Under  the  auspices  of  the  Department  of  Agriculture  at 
Washington,  considerable  effort  has  been  put  forth  in  the 

'Russell,  Bull.  69,  Wis.  Expt.  Stat. 


Bacteria  and  Butter- Making.  151 

preparation  of  culture  butter  especially  for  export  trade. 
In  some  cases  this  work  has  borne  fruitage,1  but  in  general 
the  flavor  of  butter  made  from  pasteurized  cream  is  not  as 
"  high  "  and  "  quick  "  as  that  made  in  the  other  way,  and 
therefore  it  does  not  meet  this  desired  requisite  of  the 
American  market.  The  mild,  clean  flavor  which  charac- 
terizes culture  butter  is  particularly  desired  by  the  English 
market,  to  which  the  bulk  of  the  Danish  butter  goes.  Con- 
siderable " pasteurized "  or  "culture"  butter  has  been  ex- 
ported from  Australia 2  and  New  Zealand  to  England  and 
it  is  said  that  the  system  is  gaining  ground  slowly. 

Where  the  market  demands  are  satisfied  with  the  quality 
of  butter  that  can  best  be  made  by  this  system,  the  method 
is  undoubtedly  destined  to  be  adopted  more  and  more  gen- 
erally, as  the  uniformity  of  product  obtained  is  a  great  ad- 
vantage. The  system  entails  considerable  labor  and  some 
expense,  and  the  question  as  to  its  more  general  adoption 
will  be  determined  by  the  advantages  gained. 

Propagation  of  starters  for  cream-ripening:.  The  prepa- 
ration-and  propagation  of  a  starter  for  cream-ripening  is  a 
process  involving  considerable  bacteriological  knowledge, 
whether  the  starter  is  of  domestic  origin  or  prepared  from 
a  pure-culture  ferment.  In  any  event,  it  is  necessary  that 
the  starter  should  be  handled  in  a  way  so  as  to  prevent  the 
introduction  of  foreign  bacteria  as  far  as  possible.  The 
following  points  should  be  kept  in  mind  in  manipulating 
the  starter: 

1.  If  a  pure-culture  ferment  is  used,  see  that  it  is  fresh 
and  that  the  seal  has  not  been  disturbed. 

1  Some  of  the  larger  western  creamery  syndicates  are  pasteurizing  a  large 
part  of  the  cream  they  receive. 

2  Cherry,  Farm  and  Home,  Aug.,  1900. 


152  Dairy  Bacteriology. 

2.  If  a  home-made  starter  is  employed,  use  the  greatest 
possible  care  in  selecting  the  milk  that  is  to  be  used  as  a 
basis  for  the  starter. 

3.  For  the  propagation  and  perpetuation  of  the  starter 
from  day  to  day,  it  is  necessary  that  the  same  should  be 
grown  in  milk  that  is  as  germ-free  as  it  is  possible  to  secure 
it.     For  this  purpose  sterilize  some  fresh  skim-milk  in  a 
covered  can  that  has  previously  been  well  steamed.     This 
can  be  done  easily  by  setting  cans  containing  skim-milk  in 
a  vat  filled  with  water  and  heating  th*  same  to  180°  F.  or 
above.     The  temperature  should  be  maintained  for  a  half 
hour  or  more.     This  destroys  all  but  a  few  of  the  most  re- 
sistant spore-bearing  organisms.     This  will  give  a  cooked 
flavor  to  the  milk,  but  will  not  affect  the  cream  to  which 
the  starter  is  added.     Dairy  supply  houses  are  now  intro- 
ducing the  use  of  starter  cans  that  are  specially  made  for 
this  purpose. 

4.  After  the  heated  milk  is  cooled  down  to  about  70°  or 
80°  F.,  it  can  be  inoculated  with  the  desired  culture.   Some- 
times it  is  desirable  to  "  build  up  "  the  starter  by  propagat- 
ing it  first  in  a  smaller  volume  of  milk,  and  then  after  this 
has  developed,  adding  it  to  a  larger  amount. 

This  method  is  of  particular  value  where  a  large  amount 
of  starter  is  needed  for  the  cream-ripening. 

5.  After  the  milk  has  been  inoculated,  it  should  be  kept 
at  a  temperature  that  is  suitable  for  the  rapid  development 
of  the  contained  bacteria,  65°-75°  F.,  which  temperature 
should  be  kept  as  uniform  as  possible. 

6.  This  can  best  be  done  by  setting  can  in  vat  filled  with 
warm  water  and  covering  the  same  with  a  wooden  cover 
or  heavy  cloth  during  the  night  to  maintain  proper  tem- 
perature. 


Bacteria  and  Butter- Making.  153 

7.  The  starter  should  not  be  thoroughly  curdled  and  solid 
when  it  is  needed  for  use,  but  should  be  well  soured  and 
partially  curdled.     This  point  is  of  importance  for  the  fol- 
lowing reasons: 

a.  It  is  difficult  to  thoroughly  break  up  curd  particles  if 
the  starter  is  completely  curdled.   If  these  curd  masses  are 
added  to  ripening  cream,  white  specks  may  appear  in  the 
butter. 

b.  The  vigor  of  the  starter  is  in  all  probability  stronger 
when  the  milk  is  on  the  point  of  curdling  than  it  is  after 
the  curd  has  been  formed  some  time.     The  continued  for- 
mation of  lactic  acid  kills  many  of  the  bacteria  and  thus 
weakens  the  fermentative  action.     It  is  therefore  highly 
important  that  the  acidity  of  the  starter  should  be  closely 
watched. 

8.  The  starter  should  be  propagated  from  day  to  day  by 
adding  a  small  quantity  to  a  new  lot  of  freshly  prepared 
milk.     For  this  purpose  two  propagating  cans  should  be 
provided  so  that  one  starter  may  be  in  use  while  the  other 
is  being  prepared. 

9.  The  butter-maker  must  exercise  his  judgment  as  to 
the  condition  of  his  starter.     If  the  same  should  appear 
moldy  or  contain  evidence  of  gas,  the  skim  milk  has  been 
imperfectly  handled. 

How  long  should  a  starter  be  propagated?  No  hard-and- 
fast  rule  can  be  given  for  this,  for  it  depends  largely  upon 
how  carefully  the  starter  is  handled  during  its  propaga- 
tion. If  the  starter  is  grown  in  sterilized  milk  kept  in 
steamed  vessels  and  is  handled  with  sterile  dippers,  it  is 
possible  to  maintain  it  in  a  state  of  relative  purity  for  a 
considerable  period  of  time;  if,  however,  no  especial  care  is 
given,  it  will  soon  become  infected  by  the  air,  and  the 


154:  Dairy  Bacteriology. 

retention  of  its  purity  will  depend  more  upon  the  ability 
of  the  contained  organism  to  choke  out  foreign  growths 
than  upon  any  other  factor.  Experience  seems  to  indicate 
that  pure-culture  starters  "  run  out "  sooner  than  domestic 
starters.  While  it  is  possible,  by  bacteriological  methods, 
to  determine  with  accuracy  the  actual  condition  of  a  starter 
as  to  its  germ  content,  still  such  methods  are  inapplicable 
in  creamery  practice.  Here  the  maker  must  rely  largely 
upon  the  general  appearance  of  the  starter  as  determined 
by  taste  and  smell.  The  supply  houses  that  deal  in  cult- 
ures of  this  class  generally  expect  to  supply  a  new  culture 
at  least  every  month. 

Bacteria  in  butter.  As  ripened  cream  is  necessarily  rich 
in  bacteria,  it  follows  that  butter  will  also  contain  germ 
life  in  varying  amounts,  but  as  butter- fat  is  not  well  adapted 
for  bacterial  food,  the  number  of  germs  in  butter  is  usually 
less  than  in  ripened  cream. 

Sweet-cream  butter  is  naturally  poorer  in  germ  life  than 
that  made  from  ripened  cream.  Grotenfelt1  reports  in 
sweet-cream  butter,  the  so-called  "  Paris  butter,"  only 
120  to  300  bacteria  per  cc.,  while  in  butter  from  sour  crearn 
2,000  to  55,000  germs  per  cc.  were  found.  Pammel8  found 
from  125,000  to  730,000  per  gram,  while  Lafar3  found  in 
butter  sold  in  Munich  from  10,000,000  to  20,000,000  organ- 
isms per  gram. 

The  germ  content  of  butter  on  the  outside  of  a  package 
is  much  greater  than  it  is  in  the  middle  of  a  mass,  this 
doubtless  being  due  to  the  freer  access  of  air  favoring  the 
growth  of  aerobic  forms. 

» Grotenfelt-Woll,  Prin.  Mod.  Dairy  Practice,  p.  244. 
«  Pammel,  Bull.  21,  Iowa  Expt.  Stat.,  p.  801. 
«  Lafar,  Arch.  f.  Hyg.,  1891,  18:1. 


Bacteria  and  Butter- Making.  155 

Changes  in  germ  content.  The  bacteria  that  are  incor- 
porated with  the  butter  as  it  first  "comes"  undergo  a 
slight  increase  for  the  first  few  days.  The  duration  of  this 
period  of  increase  is  dependent  largely  upon  the  condition 
of  the  butter.  If  the  buttermilk  is  well  worked  out  of  the 
butter,  the  increase  is  slight  and  lasts  for  a  few  days  only, 
while  the  presence  of  so  nutritious  a  medium  as  buttermilk 
affords  conditions  much  more  favorable  for  the  continued 
growth  of  the  organisms. 

While  there  may  be  many  varieties  in  butter  when  it  is 
fresh,  they  are  very  soon  reduced  in  kind  as  well  as  num- 
ber. The  lactic  acid  group  of  orgariisms  disappear  quite 
rapidly;  the  sppre-bearing  species  remaining  for  a  some- 
what longer  time.  Butter  examined  after  it  is  several 
months  old  is  often  found  to  be  almost  free  from  germs. 

In  the  manufacture  of  butter  there  is  much  that  is  de- 
pendent upon  the  mechanical  processes  of  churning,  wash- 
ing, salting  and  working  the  product.  These  processes  do 
not  involve  any  bacteriological  principles  other  than  those 
that  are  incident  to  cleanliness.  The  cream,  if  ripened 
properly,  will  contain  such  enormous  numbers  of  favorable 
forms  that  the  access  of  the  few  organisms  that  are  derived 
from  the  churn,  the  air,  or  the  water  in  washing  will  have 
little  effect,  unless  the  conditions  are  abnormal. 

Rancid  change  in  butter.  Fresh  butter  has  a  peculiar 
aroma  that  is  very  desirable  and  one  that  enhances  the 
market  price,  if  it  can  be  retained;  but  this  delicate  flavor 
is  more  or  less  evanescent,  soon  disappearing,  even  in  the 
best  makes.  While  a  good  butter  loses  with  age  some  of 
the  peculiar  aroma  that  it  possesses  when  first  made,  yet  a 
gilt-edged  product  should  retain  its  good  keeping  qualities 
for  some  length  of  time.  All  butters,  however,  sooner  or 


15G  Dairy  Bacteriology. 

later  undergo  a  change  that  renders  them  worthless  for  table 
use.  This  change  is  usually  a  rancidity  that  is  observed 
in  all  stale  products  of  this  class.  The  cause  of  this  rancid 
condition  in  butter  has  been  attributed  to  the  action  of 
living  organisms,  particularly  those  that  form  butyric  acid, 
to  the  influence  of  light,  air,  etc.  Undoubtedly  under  cer- 
tain conditions,  rancidity  may  be  produced  by  the  opera- 
tion of  all  of  the  above  agents.  Although  the  subject  has 
been  quite  extensively  studied,  there  is  yet  considerable 
variation  in  opinion  as  to  the  exact  nature  of  the  causal 
agents.1 

BACTERIAL  DEFECTS  IN"  BUTTER. 

Lack  of  flavor.  Often  this  may  be  due  to  improper 
handling  of  the  cream  in  not  allowing  it  to  ripen  far 
enough,  but  sometimes  it  is  impossible  to  produce  a  high 
flavor.  The  lack  of  flavor  in  this  case  is  due  to  the  ab- 
sence of  the  proper  flavor-producing  organisms.  This  con- 
dition can  usually  be  overcome  by  the  addition  of  a  proper 
starter.  The  relation  between  flavor  and  desirable  bacteria 
is  very  intimate,  and  troubles  of  this  kind  usually  arise 
because  the  proper  forms  commonly  found  in  the  cream 
have  been  supplanted  by  other  species  that  do  not  possess 
the  ability  of  forming  these  aromatic  substances  so  neces- 
sary in  sour-cream  butter. 

Putrid  butter.  This  specific  butter  trouble  has  been  ob- 
served in  Denmark,  where  it  has  been  studied  by  Jensen.8 
Butter  affected  by  it  rapidly  acquires  a  peculiar  putrid 
odor  that  ruins  it  for  table  use.  Sometimes,  this  flavor 
may  be  developed  in  the  cream  previous  to  churning. 

iReinmann.  Cent.  f.  Bakt.,  1900,  6:131;  Jensen,  Landw.  Jahr.  d.  Schweiz,  1901. 
•Jensen,  Cent.  f.  Bakt.,  1891, 11:409. 


Bacteria  and  Butter- Making.  157 

Jensen  found  the  trouble  to  be  due  to  several  different 
putrefactive  bacteria.  One  form  which  he  called  Bacillus 
fcetidus  lactis,  a  close  ally  of  the  common  feces  bacillus, 
produced  this  rotten  odor  and  taste  in  milk  in  a  very  short 
time.  Fortunately,  this  organism  was  easily  killed  by  a 
comparatively  low  heat,  so  that  pasteurization  of  the  cream 
and  use  of  a  culture  starter  quickly  eliminated  the  trouble, 
where  it  was  tried. 

Turnip-flavored  butter.  Butter  sometimes  acquires  a 
peculiar  flavor  recalling  the  order  of  turnips,  rutabagas, 
and  other  root  crops.  Often  this  trouble  is  due  to  feed- 
ing, there  being  in  several  of  these  crops,  aromatic  sub- 
stances that  pass  directly  into  the  milk,  but  in  some  in- 
stances the  trouble  arises  from  bacteria  that  are  able  to 
produce  decomposition  products,1  the  odor  and  taste  of 
which  strongly  recalls  these  vegetables. 

"Cowy"  butter.  Frequently  there  is  to  be  noted  in 
milk  a  peculiar  odor  that  resmbles  that  of  the  cow  stable. 
Usually  this  defect  in  milk  has  been  ascribed  to  the  absorp- 
tion of  impure  gases  by  the  milk  as  it  cools,  although  the 
gases  and  odors  naturally  present  in  fresh  mills:  have  this 
peculiar  property  that  is  demonstrable  by  certain  methods 
of  aeration.  Occasionally  it  is  transmitted  to  butter,  and 
recently  Pammel"  has  isolated  from  butter  a  bacillus  that 
produced  in  milk  the  same  peculiar  odor  so  commonly  pres- 
ent in  stables. 

Lardy  and  tallowy  butter.  The  presence  of  this  un- 
pleasant taste  in  butter  may  be  due  to  a  variety  of  causes. 
In  some  instances,  improper  food  seems  to  be  the  source  of 
the  trouble;  then  again,. butter  exposed  to  direct  sunlight 

»  Jensen,  Milch  Zeit.,  1892,  6,  Nos.  5  and  6. 

Bull.  21,  Iowa  Expt.  Stat.,  p.  803. 


158  Dairy  Bacteriology. 

bleaches  in  color  and  develops  a  lardy  flavor.1  In  addition 
to  these,  cases  have  been  found  in  which  the  defect  has  been 
traced  to  the  action  of  bacteria.  Storch  *  has  described  a 
lactic-acid  form  in  a  sample  of  tallowy  butter  that  was  able 
to  produce  this  disagreeable  odor. 

Oily  butter.  Jensen  has  isolated  one  of  the  causes  of  the 
dreaded  oily  butter  that  is  reported  quite  frequently  in 
Denmark.  The  specific  organism  that  he  found  belongs 
to  the  sour-milk  bacteria.  In  twenty-four  hours  it  curdle8 
milk,  the  curd  being  solid  like  that  of  ordinary  sour  milk. 
There  is  produced,  however,  in  addition  to  this,  an  unpleas- 
ant odor  and  taste  resembling  that  of  machine  oil,  a  pe- 
culiarity that  is  transmitted  directly  to  butter  made  from 
affected  cream. 

Bitter  butter.  Now  and  then  butter  develops  a  bitter 
taste  that  may  be  due  to  a  variety  of  different  bacterial 
forms.  In  most  cases,  the  bitter  flavor  in  the  butter  is 
derived  primarily  from  the  bacteria  present  in  the  cream 
or  milk.  Several  of  the  fermentations  of  this  character 
in  milk  are  also  to  be  found  in  butter.  In  addition  to 
these  defects  produced  by  a  biological  cause,  bitter  flavors 
in  butter  are  sometimes  produced  by  the  milk  being  im- 
pregnated with  volatile,  bitter  substances  derived  from 
weeds. 

Moldy  butter.  This  defect  is  perhaps  the  most  serious 
because  most  common.  It  is  produced  by  the  development 
of  a  number  of  different  varieties  of  molds.  The  trouble 
appears  most  frequently  in  packed  butter  on  the  outside  of 
the  mass  of  butter  in  contact  with  the  tub.  Mold  spores 
are  so  widely  disseminated  that  if  proper  conditions  are 

1  Fischer,  Hyg.  Bund,  5:573. 

2  Storch,  18  Kept.  Danish  Agric.  Expt,  Stat.,  1890. 


Bacteria  and  Butter-Making.  159 

given  for  their  germination,  they  are  almost  sure  to  develop. 
In  some  cases  the  mold  is  due  to  the  growth  of  the  ordinary 
bread  mold,  Penicillium  glaucum;  in  other  cases  a  black 
mold  develops,  due  often  to  Cladosporium  butyri.  Not  in- 
frequently trouble  of  this  character  is  associated  with  the 
use  of  parchment  wrappers.  The  difficulty  can  easily  be 
held  in  check  by  soaking  the  parchment  linings  and  the 
tubs  in  a  strong  brine. 

Fishy  butter.  Considerable  trouble  has  been  experienced 
in  Australian  butter  exported  to  Europe  in  which  a  fishy 
flavor  developed.  It  was  noted  that  the  production  of  this 
defect  seemed  to  be  dependent  upon  the  storage  tempera- 
ture at  which  the  butter  was  kept.  When  the  butter  was 
refrigerated  at  15°  F.  no  further  difficulty  was  experienced. 
It  is  claimed  that  the  cause  of  this  condition  is  due  to  the 
formation  of  trimethylamine  (herring  brine  odor)  due  to 
the  growth  of  the  mold  fungus  Oidium  lactis,  developing 
in  combination  with  the  lactic-acid  bacteria. 


CHAPTER  VIII. 
BACTERIA  IN  CHEESE. 

THE  art  of  cheese-making,  like  all  other  phases  of  dairy- 
ing, has  been  developed  mainly  as  a  result  of  empirical 
methods.  Within  the  last  decade  or  so,  the  subject  has 
received  more  attention  from  the  scientific  point  of  view 
and  the  underlying  causes  determined  to  some  extent. 
Since  the  subject  has  been  investigated  from  the  bacterio- 
logical point  of  view,  much  light  has  been  thrown  on  the 
cause  of  many  changes  that  were  heretofore  inexplicable. 
Our  knowledge,  as  yet,  is  quite  meager,  but  enough  has 
already  been  determined  to  indicate  that  the  whole  indus- 
try is  largely  based  on  the  phenomena  of  ferment  action,  and 
that  the  application  of  bacteriological  principles  and  ideas 
is  sure  to  yield  more  than  ordinary  results,  in  explaining, 
in  a  rational  way,  the  reasons  underlying  many  of  the  pro- 
cesses to  be  observed  in  this  industry. 

The  problem  of  good  milk  is  a  vital  one  in  any  phase  of 
dairy  activity,  but  it  is  pre-eminently  so  in  cheese-making, 
for  the  abilitjr  to  make  a  first-class  product  depends  to  a 
large  extent  on  the  quality  of  the  raw  material.  Cheese 
contains  so  large  a  proportion  of  nitrogenous  constituents 
that  it  is  admirably  suited,  as  a  food  medium,  to  the  devel- 
opment of  bacteria;  much  better,  in  fact,  than  butter. 

INFLUENCE  OF  BACTERIA  IN  NORMAL  CHEESE  PROCESSES . 

In  the  manufacture  of  cheddar  cheese  bacteria  exert  a 
marked  influence  in  the  initial  stages  of  the  process.  To 
produce  the  proper  texture  that  characterizes  cheddar 
cheese,  it  is  necessary  to  develop  a  certain  amount  of  acid 


Bacteria  in  Cheese.  1611 

which  acts  upon  the  casein.  This  acidity  is  measured  by> 
the  development  of  the  lactic-acid  bacteria  that  normally 
abound  in  the  milk;  or,  as  the  cheese-maker  expresses  it, 
the  milk  is  "ripened  "  to  the  proper  point.  The  action  of 
the  rennet,  which  is  added  to  precipitate  the  casein  of  the 
milk,  is  markedly  affected  by  the  amount  of  acid  present, 
as  well  as  the  temperature.  Hence  it  is  desirable  to  have 
a  standard  amount  of  acidity  as  well  as  a  standard  tem- 
perature for  coagulation,  so  as  to  unify  conditions.  It 
frequently  happens  that  the  milk  is  abnormal  with  refer- 
ence to  its  bacterial  content,  on  account  of  the  absence  of 
the  proper  lactic  bacteria,  or  the  presence  of  forms  capable 
of  producing  fermentative  changes  of  an  undesirable  char- 
acter. In  such  cases  the  maker  attempts  to  overcome  the 
effect  of  the  unwelcome  bacteria  by  adding  a  "starter;  "  or. 
he  must  vary  his  method  of  manufacture  to  some  extent 
to  meet  these  new  conditions. 

Use  of  Starters.  A  starter  maybe  employed  to  hasten 
the  ripening  of  milk  that  is  extremely  sweet,  so  as  to  cur- 
tail the  time  necessary  to  get  the  cheese  to  press;  or  it 
may  be  used  to  overcome  the  effect  of  abnormal  conditions. 

The  starter  that  is  employed  is  generally  one  of  domestic 
origin,  and  is  usually  taken  from  skim  milk  that  has  been 
allowed  to  ferment  and  sour  under  carefully  controlled  con- 
ditions. Of  course  much  depends  upon  the  quality  of  the 
starter,  and  in  a  natural  starter  there  is  always  the  possibility 
that  it  may  not  be  perfectly  pure. 

Within  recent  years  the  attempt  has  been  made  to  con- 
trol the  effect  of  the  starter  more  thoroughly  by  using  pure 
cultures  of  some  desirable  lactic-acid  form.1  This  has  ren- 

1  Russell,  13  Rept.  Wis.  Expt.  Stat.,  1896,  p.  112;  Campbell,  Trans.  High.  & 
Agr.  Soc.  Scotland,  5  ser.,  1898,  10:181. 
11 


162  Dairy  Bacteriology. 

dered  the  making  of  cheese  not  only  more  uniform,  but  has 
aided  in  repressing  abnormal  fermentations  particularly 
those  that  are  characturized  by  the  production  of  gas. 

Recentl3T,  pure  cultures  of  Adametz's  B.  nobilis,  a  di- 
gesting organism  that  is  claimed  to  be  the  cause  of  the 
breaking  down  of  the  casein  and  also  of  the  peculiar  aroma 
of  Emmenthaler  cheese,  has  been  placed  on  the  market 
under  the  name  Tyrogen.  It  is  claimed  that  the  use  of  this 
starter,  which  is  added  directly  to  the  milk  and  also  rubbed 
on  the  surface  of  the  cheese,  results  in  the  improvement  of 
the  curds,  assists  in  the  development  of  the  proper  holes, 
imparts  a  favorable  aroma  and  hastens  ripening.1 

Campbell 2  states  that  the  discoloration  of  cheese  in  Eng- 
land, which  is  due  to  the  formation  of  white  spots  that  are 
produced  by  the  bleaching  of  the  coloring  matter  in  the 
cheese,  may  be  overcome  by  the  use  of  lactic-acid  starters. 

The  use  of  stringy  or  slimy  whey  has  been  advocated  in 
Holland  for  some  years  as  a  means  of  overcoming  the 
tendency  toward  gas  formation  in  Edam  cheese  which  is 
made  from  practically  sweet  milk.  This  fermentation,  the 
essential  feature  of  which  is  produced  by  a  culture  oi  Strep- 
tococcus Hollandicus?  develops  acid  in  a  marked  degree, 
thereby  inhibiting  the  production  of  gas. 

The  use  of  masses  of  moldy  bread  in  directing  the  fer- 
mentation of  Roquefort  cheese  is  another  illustration  of  the 
empirical  development  of  starters,  although  in  this  in- 
stance it  is  added  after  the  curds  have  been  prepared  for 
the  press. 

Pasteurizing  milk  for  cheese-making;.  If  it  were  pos- 
sible to  use  properly  pasteurized  milk  in  cheese-making, 

* Winkler,  Milch  Zeit.  (Hildesheim},  Nov.  24,  1900. 
'Campbell,  No.  Brit.,  Agric.,  May  12,  1897.  , 
•  Weigmann,  Milch  Zeit.,  No.  50,  1889. 


J3acteria  in  Cheese.  163 

then  practically  all  abnormal  conditions  could  be  controlled 
by  the  use  of  properly  selected  starters.  Numerous  at- 
tempts have  been  made  to  perfect  this  system  with  refer- 
ence to  cheddar  cheese,  but  so  far  they  have  been  attended 
with  imperfect  success.  The  reason  for  this  is  that  in  pas- 
teurizing milk,  the  soluble  lime  salts  are  precipitated  by  the 
action  of  heat,  and  under  these  conditions  rennet  extract 
does  not  curdle  the  casein  in  a  normal  manner.  This  con- 
dition can  be  restored,  in  part  at  least,  by  the  addition  of 
soluble  lime  salts,  such  as  calcium  chlorid;  but  in  our  ex- 
perience, desirable  results  were  not  obtained  where  heated 
milks  to  -which  this  calcium  solution  had  been  added  were 
made  into  cheddar  cheese.  Considerable  experience  has  been 
gained  in  the  use  of  heated  milks  in  the  manufacture  of  cer- 
tain types  of  foreign  cheese.  Klein1  finds  that  Brick  cheese 
•can  be  successfully  made  even  where  the  milk  is  heated  as 
high  as  185°  F.  An  increased  weight  is  secured  by  the  addi- 
tion of  the  coagulated  albumin  and  also  increased  moisture. 
Bacteria  in  rennet.  In  the  use  of  natural  rennets,  such 
as  are  frequently  employed  in  tbe  making  of  Swiss  cheese, 
considerable  numbers  of  bacteria  are  added  to  the  milk.  Al- 
though these  rennets  are  preserved  in  salt,  alcohol  or  boric 
acid,  they  are  never  free  from  bacteria.  Adametz 8  found 
ten  different  species  and  from  640,000  to  900,000  bacteria 
per  cc.  in  natural  rennets.  Freudenreich  has  shown  that 
rennet  extract  solutions  can  be  used  in  Swiss  cheese-mak- 
ing quite  as  well  as  natural  rennets;  but  to  secure  the  best 
results,  a  small  quantity  of  pure  lactic  ferment  must  be 
added  to  simulate  the  conditions  that  prevail  when  natural 
rennets  are  soaked  in  whey,  which,  it  must  be  remembered, 
is  a  fluid  rich  in  bacterial  life. 

*  Klein,  Milch  Zeit.  (Hildesheim),  No.  17,  1900. 
8  Adametz,  Landw.  Jahr.,  18:256. 


164  Dairy  Bacteriology. 

Where  rennet  extract  or  tablets  are  used,  as  is  gener- 
erally  the  case  in  cheddar  making,  the  number  of  bacteria 
added  is  so  infinitesimal  as  to  be  negligible. 

Development  Of  acid.  In  the  manufacture  of  cheddar 
cheese,  the  development  of  acid  exerts  an  important  influ- 
ence on  the  character  of  the  product.  This  is  brought 
about  by  holding  the  curds  at  temperatures  favorable  to  the 
growth  of  the  bacteria  in  the  same.  Under  these  conditions 
the  lactic-acid  organisms,  which  usually  predominate,  de- 
velop very  rapidly,  producing  thereby  considerable  quan- 
tities of  acid  which  change  materially  the  texture  of  the 
curds.  This  acid  unites  to  some  extent  with  the  casein, 
thereby  producing  compounds  of  a  character  different  from 
those  existing  in  the  green  curds.  The  acid  also  ex- 
erts a  slight  solvent  effect  on  the  casein,  as  is  seen  in  the 
"  strings  "  made  on  the  "  hot  iron."  This  causes  the  curds 
to  mat,  producing  a  close,  solid  body  free  from  mechan- 
ical holes.  Still  further,  the  development  of  this  acid  is 
necessary  for  the  digestive  activity  of  the  pepsin  in  the 
rennet  extract. 

In  some  varieties  of  cheese,  as  the  Swiss,  acid  is  not  de- 
veloped and  the  character  of  the  cheese  is  much  different 
from  that  of  cheddar.  In  all  such  varieties,  a  great  deal 
more  trouble  is  experienced  from  the  production  of  "  gassy  " 
curds,  because  the  development  of  the  gas-producing  bac- 
teria is  held  in  check  by  the  rapid  growth  of  the  lactic  acid- 
producing  species. 

Bacteria  in  green  cheese.  The  conditions  under  which 
cheese  is  made  permit  of  the  development  of  bacteria 
throughout  the  entire  process.  The  cooking  or  heating  of 
curds  to  expel  the  excessive  moisture  is  never  so  high  as  to 
be  fatal  to  germ  life;  on  the  contrary,  the  acidity  of  the 


Bacteria  .in  Cheese. 


165 


curd  and  whey  is  continually  increased  by  the  development 
of  bacteria  in  the  same. 

The  body  of  green  cheese  fresh  from  the  press  is,  to  a 
considerable  extent,  dependent  upon  the  acid  produced  in 
the  curds.  If  the  curds  are  put  to  press  in  a  relatively 
sweet  condition  the  texture  is  open  and  porous.  The  curd 
particles  do  not  mat  closely  together  and  u  mechanical 
holes,"  rough  and  irregular  in  outline,  occur.  Very  often, 


FIG.  30.  L,  a  sweet  curd  cheese  direct  from  the  press.  "  Mechanical "  holes  due 
to  lack  of  acid  development;  P,  same  cheese  fpur  days  later,  mechanical  holes 
distended  by  development  of  gas. 

at  relatively  high  temperatures,  such  cheese  begin  to 
u  huff,"  soon  after  being  taken  from  the  press,  a  condition 
due  to  the  development  of  gas,  produced  by  gas-generating 
bacteria  acting  on  the  sugar  in  the  curd.  This  gas  finds 
its  way  readily  into  these  ragged  holes,  greatly  distending 
them,  as  in  Fig.  30. 

Physical  changes  in  ripening  cheese.  When  a  green 
cheese  is  taken  from  the  press,  the  curd  is  tough,  firm,  but 
elastic.  It  has  no  value  as  a  food  product  for  immediate  use, 


166  Dairy  Bacteriology. 

because  it  lacks  a  desirable  flavor  and  is  not  readily  digesti- 
ble. It  is  nothing  but  precipitated  casein  and  fat.  in  a 
short  time,  a  deep-seated  change  occurs.  Physically  this 
change  is  demonstrated  in  the  modification  that  the 
curd  undergoes.  Gradually  it  breaks  down  and  becomes 
plastic,  the  elastic,  tough  curd  being  changed  into  a  soft- 
ened mass.  This  change  in  texture  of  the  cheese  is  also  ac- 
companied by  a  marked  change  in  flavor.  The  green  cheese 
has  no  distinctively  cheese  flavor,  but  in  course  of  time, 
with  the  gradual  change  of  texture,  the  peculiar  flavor  in- 
cident to  ripe  cheese  is  developed. 

The  characteristic  texture  and  flavor  are  susceptible  of 
considerable  modification  that  is  induced  not  only  by  varia- 
tion in  methods  of  manufacture,  but  by  the  conditions 
under  which  the  chaese  are  cured.  The  amount  ot  moist- 
ure incorporated  with  the  curd  materially  affects  the  phys- 
ical appearance  of  the  cheese,  and  the  rate  of  change  in  the 
same.  The  ripening  temperature,  likewise  the  moisture 
content  of  the  surrounding  air,  also  exerts  a  marked  in- 
fluence on  the  physical  properties  of  the  cheese.  To  some 
extent  the  action  of  these  forces  is  purely  physical,  as  in 
the  gradual  loss  by  drying,  but  in  other  respects  they  are 
associated  with  chemical  transformations. 

Chemical  changes  in  ripening  cheese.  Coincident  with 
the  physical  breaking  down  of  the  curd  comes  a  change  in 
the  chemical  nature  of  the  casein.  The  hitherto  insoluble 
casein  is  gradually  transformed  into  soluble  nitrogenous 
substances  (caseone  of  Duclaux,  or  caseogluten  of  Weig- 
mann).  This  chemical  phenomenon  is  a  breaking-down 
process  that  is  analogous  to  the  peptonization  of  proteids, 
although  in  addition  to  the  peptones  and  albumoses  char- 
acteristic of  peptic  digestion,  amido-acids  and  ammonia  are 


Bacteria  in  Cheese.  167 

to  be  found.  The  quantity  of  these  lower  products  in- 
creases with  the  age  of  the  cheese. 

The  chemical  reaction  of  cheese  is  normally  acid  to 
phenolphthalein,  although  there  is  generally  no  free  acid, 
as  shown  by  Congo  red,  the  lactic  acid  being  converted 
into  salts  as  fast  as  formed.  In  very  old  cheese,  undergo- 
ing putrefactive  changes,  especially  on  the  outside,  an  alka- 
line reaction  may  be  present,  due  to  the  formation  of  free 
ammonia. 

The  changes  that  occur  in  a  ripening  cheese  are  for  the 
most  part  confined  to  the  proteids.  According  to  most  in- 
vestigators the  fat  remains  practically  unchanged,  although 
the  researches  of  Weigrnann  and  Backe l  show  that  fatty 
acids  are  formed  from  the  fat.  In  the  green  cheese  con- 
siderable milk-sugar  is  present,  but,  as  a  result  of  the  fer- 
mentation that  occurs,  this  is  rapidly  converted  into  acid 
products. 

Bacterial  flora  Of  Cheese.  It  might  naturally  be  expected 
that  the  green  cheese,  fresh  from  the  press,  would  contain 
practically  the  same  kind  of  bacteria  that  are  in  the  milk, 
but  a  study  of  cheese  shows  a  peculiar  change  in  the  char- 
acter of  the  flora.  In  the  first  place,  fresh  cottage  cheese, 
made  by  the  coagulation  of  the  casein  through  the  action 
of  acid,  has  a  more  diversified  flora  than  cheese  mada  with 
rennet,  for  the  reason,  as  given  by  Lafar,2  that  the  fermen- 
tative process  is  farther  advanced. 

When  different  varieties  of  cheese  are  made  from  milk  in 
the  same  locality,  the  germ  content  of  even  the  ripened 
product  has  a  marked  similarity,  as  is  illustrated  by 
Adametz's  work 3  on  Emmenthaler  or  Swiss  hard  cheese, 

1  Milch  Zeit.,  1898,  No.  49, 

2  Lafar,  Technical  Mycology,  p.  21t>. 
8  Adametz,  Landw.  Jahr.,  18: 228. 


168  Dairy  Bacteriology. 

and  Schweitzer  Hauskase,  a  soft  variety.  Of  the  nln3 
species  of  bacilli  and  cocci  found  in  mature  Emmenthaler, 
eight  of  them  were  also  present  in  ripened  Hauskase. 

Different  investigators  have  studied  the  bacterial  flora  of 
various  kinds  of  cheese,  but  as  yet  little  comparative  sys- 
tematic work  has  been  done.  Freudenreich  *  has  determined 
the  character  and  number  of  bacteria  in  Emmenhtaler 
cheese,  and  Russell 2  the  same  for  cheddar  cheese.  The  same 
general  law  has  also  been  noted  in  Canadian 8  and  Eng- 
lish4 cheese.  At  first  there  is  found  a  marked  decrease  in 
numbers,  lasting  for  a  day  or  two.  This  is  followed  by  an 
enormous  increase,  caused  by  the  rapid  growth  of  the 
lactic-acid  type.  The  development  may  reach  scores  oi 
millions  and  often  over  a  hundred  million  organisms  per 
gram.  Synchronous  with  this  increase,  the  peptonizing 
and  gas-producing  bacteria  gradually  disappear.  This  rapid 
development,  which  lasts  only  for  a  few  weeks,  is  followed 
by  a  general  decline. 

In  the  ripening  of  cheese  a  question  arises  as  to  whether 
the  process  goes  on  throughout  the  entire  mass  of  cheese, 
or  whether  it  is  more  active  at  or  near  the  surface.  In  the 
case  of  many  of  the  soft  cheese,  such  as  Brie  and  limburger, 
bacterial  and  mold  development  is  exceedingly  active  on  the 
exterior,  and  the  enzyms  secreted  by  these  organisms  dif- 
fuse toward  the  interior.  That  such  a  condition  occurs  in 
the  hard  type  of  cheese  made  with  rennet  is  extremely  im- 
probable. Most  observers  agree  that  in  this  type  of  cheese 
the  ripening  progresses  throughout  the  entire  mass,  al- 
though Adametz  opposes  this  view  and  considers  that  in 

1  Freudenreich,  Landw.  Jahr.  d.  Schweiz,  4: 17;  5: 16. 

'Russell,  13  Kept.  Wis.  Expt.  Stat.,  18%,  p.  95. 

8  Harrison,  Unpublished  Data. 

*  Lloyd,  Bath  ajd  West  of  Eng.  Soc.  Kept.,  1892,  2: 180. 


Bacteria  in  Cheese.  169 

Emmenthaler  cheese  the  development  of  the  specific  aroma- 
producing  organism  occurs  in  the  superficial  layers.  Jen- 
sen has  shown,  however,  that  the  greatest  amount  of 
soluble  nitrogenous  products  are  to  be  found  in  the  inner- 
most part  of  the  cheese,  a  condition  that  is  not  reconcil- 
able with  the  view  that  the  most  active  ripening  is  on  the 
exterior.1 

The  course  of  development  of  bacteria  in  cheddar  cheese 
is  somewhat  influenced  by  the  ripening  temperature.  In 
cheese  ripened  at  relatively  low  temperatures  (50°-55°  F.),2 
the  bacterial  flora  retains  for  a  considerable  period  the 
same  general  aspect  as  in  the  milk.  Under  these  condi- 
tions the  lactic-acid  type  does  not  gain  the  ascendancy  so 
readily.  In  cheese  cured  at  high  temperatures  (80°-86°  F.), 
the  number  of  organisms  is  greatly  diminished,  and  they 
fail  to  persist  in  appreciable  numbers  for  as  long  a  time  as 
in  cheese  cured  at  temperatures  more  frequently  employed. 

Influence  of  temperature  on  curing.  Temperature  exerts 
a  most  potent  influence  on  the  quality  of  the  cheese,  as  de- 
termined not  only  by  the  rate  of  ripening  but  the  nature 
of  the  process  itself.  Much  of  the  poor  quality  of  cheese 
is  attributable  to  the  effect  of  improper  curing  conditions. 
Probably  in  the  initial  stage  of  this  industry  cheese  were 
allowed  to  ripen  without  any  sort  of  control,  with  the 
inevitable  result  that  during  the  summer  months  the  tem- 
perature generally  fluctuated  so  much  as  to  impair  seriously 
the  quality.  The  effect  of  high  temperatures  (70°  F.  and 
above)  is  to  produce  a  rapid  curing,  and,  therefore,  a  short 
lived  cheese;  also  a  sharp,  strong  flavor,  and  generally  a 

i  Freudenreich,  Landw.  Jahr.  d.  Schweiz,  1900;  Adametz,  Oest.  Molk.  Zeit., 
1899,  No.  7.    - 
»  Russell,  14  Wis.  Expt.  Stat.,  1897,  p.  203. 


170 


Dairy  Bacteriology. 


more  or  less  open  texture.  Unless  the  cheese  is  made 
from  the  best  quality  of  milk,  it  is  very  apt  to  undergo 
abnormal  fermentations,  more  especially  those  of  a  gassy 
character. 

Where  cheese  is  ripened  at  low  temperatures,  ranging 
from  50°  F.  down  to  nearly  the  freezing  temperatures,  it  is 


9ozs. 

FIG.  31.    Influence  of  curing  temperature  on  texture  of  cheese.    Upper  row 
ripened  eight  months  at  60°  F. ;  lower  row  at  40°  F. 

found  that  the  quality  is  greatly  improved.1  Such  cheese 
are  thoroughly  broken  down  from  a  physical  point  of  view 
even  though  they  majr  not  show  such  a  high  per  cent  of 
soluble  nitrogenous  products.  They  have  an  excellent 
texture,  generally  solid  and  firm,  free  from  all  tendency  to 
openness;  and,  moreover,  their  flavor  is  clean  and  entirely 
devoid  of  the  sharp,  undesirable  tang  that  so  frequently 
appears  in  old  cheese.  The  keeping  quality  of  such  cheese 
is  much  superior  to  the  ordinary  product.  The  introduc- 

» Babcock  and  Russell,  18  Kept.  Wis.  Expt.  Stat.,  1901. 


Bacteria  in  Cheese.  171 

tion  of  this  new  system  of  cheese-curing  promises  much 
from  a  practical  point  of  view,  and  undoubtedly  a  more 
complete  study  of  the  subject  from  a  scientific  point  of  view 
will  aid  materially  in  unraveling  some  of  the  problems  as 
to  flavor  production. 

Theories  of  cheese  curing.  Within  the  last  few  years 
considerable  study  has  been  given  the  subject  of  cheese  cur- 
ing or  ripening,  in  order  to  explain  how  this  physical  and 
chemical  transformation  is  brought  about. 

Much  of  the  misconception  that  has  arisen  relative  to 
the  cause  of  cheese  ripening  comes  from  a  confusion  of 
terms.  In  the  ordinary  use  of  the  word,  ripening  or  cur- 
ing of  cheese  is  intended  to  signify  the  sum  total  of  all 
the  changes  that  result  in  converting  the  green  product 
as  it  comes  from  the  press  into  the  edible  substance  that  is 
known  as  cured  cheese.  As  previously  shown,  the  most 
marked  chemical  transformation  that  occurs  is  that  which 
has  to  do  with  the  peptonization  or  breaking  down  of  the 
casein.  It  is  true  that  under  ordinary  conditions  this  de- 
composition process  is  also  accompanied  with  the  forma- 
tion of  certain  flavor-producing  substances,  more  or  less 
aromatic  in  character;  but  it  by  no  means  follows  that  these 
two  processes  are  necessarily  related  to  each  other.  The 
majority  of  investigators  have  failed  to  consider  these  two 
questions  of  casein  decomposition  and  flavor  as  independ- 
ent, or  at  least  as  not  necessarily  related.  They  are  un- 
doubtedly closely  bound  together,  but  it  will  be  shown  later 
that  the  problems  are  quite  different  and  possibly  suscep- 
tible of  more  thorough  understanding  when  considered 
separately. 

In  the  earlier  theories  of  cheese  ripening  it  was  thought 
to  be  purely  a  chemical  change,  but,  with-  the  growth  of 


172  Da  try  Bacteriology. 

bacteriological  science,  evidence  was  forthcoming  that 
seemed  to  indicate  that  the  activity  of  organisms  entered 
into  the  problem.  SchafFer1  showed  that  if  milk  was  boiled 
and  made  into  cheese,  the  casein  failed  to  break  down. 
Adametz 2  added  to  green  cheese  various  disinfectants,  as 
creolin  and  thymol,  and  found  that  this  practically  stopped 
the  curing  process.  From  these  experiments  he  drew  the 
conclusion  that  bacteria  must  be  the  cause  of  the  change, 
because  these  organisms  were  killed;  but  when  it  is  con- 
sidered that  such  treatment  would  also  destroy  the  activity 
of  enzyms  as  well  as  vital  ferments,  it  is  evident  that  these 
experiments  were  quite  indecisive. 

A  determination  of  the  nature  of  the  by-products  found 
in  maturing  cheese  indicates  that  the  general  character  of 
the  ripening  change  is  a  peptonization  or  digestion  of  the 
casein. 

Until  recently  the  most  widely  accepted  views  relating 
to  the  cause  of  this  change  have  been  those  which  ascribed 
the  transformation  to  the  activity  of  micro-organisms, 
although  concerning  the  nature  of  these  organisms  there 
has  been  no  unanimity  of  opinion.  The  overwhelming 
development  of  bacteria  in  all  cheeses  naturally  gave  sup- 
port to  this  view;  and  such  experiments  as  detailed  above 
strengthened  the  idea  that  the  casein  transformation  could 
not  occur  where  these  ferment  organisms  were  destroj'ed. 

The  very  nature  of  the  changes  produced  in  the  casein 
signified  that  to  take  part  in  this  process  any  organism 
must  possess  the  property  of  dissolving  the  proteid  mole- 
cule, casein,  and  forming  therefrom  by-products  that  are 
most  generally  found  in  other  digestive  or  peptonizing 
changes  of  this  class. 

.  'Schaffer,  Milch  Zeit,  1889,  p.  146. 
'Adametz,  Land w.  Jahr.,  18:261. 


Bacteria  in  Cheese.  173 

Digestive  bacterial  theory.  The  first  theory  propounded 
was  that  of  Duclaux,1  who  in  1887  advanced  the  idea  that 
this  change  was  due  to  that  type  of  bacteria  which  is  able 
to  liquefy  gelatin,  peptonize  milk,  and  cause  a  hydrolytic 
change  in  proteids.  To  this  widely-spread  group  that  he 
found  in  cheese,  he  gave  the  generic  name  Tyrothrix 
(cheese  hairs).  According  to  him,  these  organisms  do  not 
function  directly  as  ripening  agents,  but  they  secrete  an 
enzym  or  unorganized  ferment  to  which  he  applies  the 
name  casease.  This  ferment  acts  upon  the  casein  of  milk, 
converting  it  into  a  soluble  product  known  as  caseone. 
These  organisms  are  found  in  normal  milk,  and  if  they 
function  as  casein  transformers,  one  would  naturally  expect 
them  to  be  present,  at  least  frequently,  if  not  predominat- 
ing in  the  ripening  cheese;  but  such  is  not  the  case.  In 
typical  cheddar  or  Swiss  cheese,  they  rapidly  disappear 
(p.  168),  although  in  the  moister,  softer  varieties,  they  per- 
sist for  considerable  periods  of  time.  According  to  Freud- 
enreich,  even  where  these  organisms  are  added  in  large 
numbers  to  the  curd,  they  soon  perish,  an  observation  that 
is  not  regarded  as  correct  by  the  later  adherents  to  the  di- 
gestive bacterial  theory,  as  Adametz  and  Winkler. 

Duclaux's  experiments  were  made  with  liquid  media  for 
isolation  purposes,  and  his  work,  therefore,  cannot  be  re- 
garded as  satisfactory  as  that  carried  out  with  more  modern 
technical  methods.  Recently  this  theory  has  been  revived 
by  Adametz,2  who  claims  to  have  found  in  Em  men  thaler 
cheese  a  digesting  species,  one  of  the  Tyrothrix  type,  which 
is  capable  of  peptonizing  the  casein  and  at  the  same  time 
producing  the  characteristic  flavor  of  this  class  of  cheese. 

i  Duclaux,  Le  Lait,  p.  213. 

«  Adametz,  Oest.  Molk.  Zeit.,  1900,  Nos.  16-18. 


174:  Dairy  Bacteriology. 

This  organism,  called  by  him  Bacillus  nolilis,  the  Edelpilz 
of  Emmenthaler  cheese,  has  been  subjected  to  comparative 
experiments,  and  in  the  cheese  made  with  pure  cultures  of 
this  germ  better  results  are  claimed  to  have  been  secured. 
Sufficient  experiments  have  not  as  yet  been  reported  by 
other  investigators  to  warrant  the  acceptance  of  the  claims 
made  relative  to  the  effect  of  this  organism. 

Lactic-acid  bacterial  theory.  It  has  already  been  shown 
that  the  lactic-acid  bacteria  seems  to  find  in  the  green 
cheese  the  optimum  conditions  of  development;  that  they 
increase  enormously  in  numbers  for  a  short  period,  and 
then  finally  decline.  This  marked  development,  coincident 
with  the  breaking  down  of  the  casein,  has  led  to  the  view 
which  has  been  so  ably  expounded  by  Freudenreich '  that 
this  type  of  bacterial  action  is  concerned  in  the  ripening  of 
cheese.  This  group  of  bacteria  is,  under  ordinary  condi- 
tions, unable  to  liquefy  gelatin,  or  digest  milk,  or,  in  fact, 
to  exert,  under  ordinary  conditions,  any  proteolytic  or  pep- 
tonizing  properties.  This  has  been  the  stumbling-block  to 
the  acceptance  of  this  hypothesis,  as  an  explanation  of 
the  breaking  down  of  the  casein.  Freudenreich  has  re- 
cently carried  on  experiments  which  he  believes  solve  the 
problem.  By  growing  cultures  of  these  organisms  in  milk, 
to  which  sterile,  freshly  precipitated  chalk  had  been  added, 
he  was  able  to  prolong  the  development  of  bacteria  for  a 
considerable  period  of  time,  and  as  a  result  finds  that  an 
appreciable  part  of  the  casein  is  digested;  but  this  action  is 
so  slow  compared  with  what  normally  occurs  in  a  cheese, 
that  exception  may  well  be  taken  to  this  type  of  experi- 
ment alone.  Weigmann  2  inclines  to  the  view  that  the 

1  Freudenreich,  Landw.  Jahr.  d.  Schweiz,  1897,  p.  85. 

*  Weigmann,  Cent.  f.  Bakt.,  II  Abt.,  1898,  4:593;  also  1899,  5:630. 


Bacteria  in  Cheese.  175 

lactic-acid  bacteria  are  not  the  true  cause  of  the  peptoniz- 
ing  process,  but  that  their  development  prepares  the  soil, 
as  it  were,  for  those  forms  that  are  more  directly  concerned 
in  the  peptonizing  process.  This  they  do  by  developing 
an  acid  substratum  that  renders  possible  the  more  luxuriant 
growth  of  the  aroma-producing  species.  According  to 
Goriiii,1  certain  of  the  Tyrothrix  forms  function  at  high 
temperatures  as  lactic  acid  producing  bacteria,  while  at 
lower  temperatures  they  act  as  peptonizers.  On  this  basis 
he  seeks  to  reconcile  the  discrepancies  that  appear  in  the 
experiments  of  other  investigators. 

Milk  enzym  (galactase)  theory.  In  1897  B  ibcock  and 
the  writer2  showed  experimentally  that  milk  is  digested 
spontaneously  when  treated  with  various  anaesthetics  like 
ether,  chloroform  and  benzol.  Under  these  conditions  it  w.is 
demonstrated  that  bacterial  activity  was  entirely  suppressed. 
Furthermore  the3r  showed  that  the  nature  of  the  by-prod- 
ucts produced  in  such  cases  was  identical  with  those  found 
in  a  maturing  cheese,  albumoses,  peptones,  arnido-acids  and 
ammonia  being  present  in  varying  amounts.  When  milk 
or  curd  was  heated  to  175°  F.  or  above,  or  treated  with 
strong  chemicals,  this  digestive  process  was  stopped. 

Under  these  conditions  the  only  agents  capable  of  pro- 
ducing such  changes  were  enzyms,  and  they  found  that  it 
was  possible  to  concentrate  this  milk  enzyin  in  centrifuge 
slime.  The  addition  of  these  slime  extracts  to  boiled  milk 
started  anew  the  digestive  process,  and  produced  by-prod- 
ucts identical  with  those  occurring  in  normal  cheese.  This 
enzym,  called  by  them  galactase,  on  account  of  its  origin 
in  milk,  is  somewhat  closely  related  to  the  tryptic  type 

1  Grorini,  Abs.  in  Expt.  Stat.  Rec..  11:388. 

a  Babcock  and  Cussell,  14  Kept.  Wis.  Expt.  Stat.,  1897,  p.  161. 


176  Dairy  Bacteriology. 

of  ferments,  but  yet  sufficiently  distinct  to  be  separated 
from  trypsin.  Jensen1  has  also  shown  that  the  addition  of 
pancreatic  extracts  to  cheese  accelerated  the  formation  of 
soluble  nitrogenous  products. 

The  action  of  galactase  in  milk  and  cheese  has  been  con- 
firmed by  Freudenreich2  and  Jensen,8  as  well  as  by  Ameri- 
can investigators,  and  this  euzym  is  now  quite  generally 
accepted  as  the  cause  of  the  decomposition  of  the  casein. 
Freudenreich  admits  that  it  plays  a  role  in  converting  the 
casein  into  albumose  and  peptones,  but  that  the  lactic-acid 
bacteria  are  chiefly  responsible  for  the  further  decomposi- 
tion of  the  nitrogen  to  amid  form. 

Failure  before  to  recognize  the  presence  of  galactase  in 
milk  is  attributable  t0  the  fact  that  all  attempts  to  secure 
sterile  milk  had  been  made  by  heating  the  same,  in  which 
case  galactase  was  necessarily  destroyed.  A  brief  exposure 
at  176°  F.  is  sufficient  to  destroy  its  activity,  and  even  an 
exposure  at  lower  temperatures  weakens  its  action  consider- 
ably, especially  if  the  reaction  of  the  medium  is  acid.  This 
undoubtedly  explains  the  contradictory  results  obtained  in 
the  ripening  of  cheese  from  pasteurized  milk,  such  cheese 
occasionally  breaking  down  in  an  abnormal  manner. 

The  results  mentioned  on  page  172,  in  which  cheese  failed 
to  ripen  when  treated  with  disinfectants, —  experiments 
which  were  supposed  at  that  time  to  be  the  foundation  of 
the  bacterial  theory  of  casein  digestion  —  are  now  explica- 
ble on  an  entirely  different  basis.  In  these  cases  the  casein 
was  not  peptonized,  because  these  strong  disinfectants  de- 
stroyed the  activity  of  the  enzyms  as  well  as  the  bacteria. 

»  Jensen,  Cent  f.  Bact.  II  Abt.,  3:752. 

•  Freudenreich,  Cent,  f .  Bakt.,  It  Abt.,  1900,  6:882. 

•  Jensen,  Ibid.,  1900,  0:734. 


Bacteria  in  Cheese.  177 

Influence  of  rennet  on  ripening.  The  addition  of  in- 
creased quantities  of  rennet  extract  facilitate  the  breaking 
down  of  the  casein  in  cheese.  This  is  due  to  the  fact  that 
such  extracts  always  contain  pepsin,  the  digestive  enzym 
found  in  the  stomach.  This  enzym  exerts  a  digestive  ef- 
fect on  the  casein  of  cheese,  producing  those  decomposi- 
tion products  (albumoses  and  higher  peptones)  that  charac- 
terize peptic  action.  This  effect  is  dependent  upon  the 
acidity  of  the  milk,  and  does  not  become  marked  until 
there  is  about  0.3  per  cent  of  lactic  acid,  which  is  about 
the  amount  developed  in  the  cheddar  process.  The  effect 
of  the  rennet  then  is  to  aid  the  galactase  in  the  peptoniza- 
tion  of  the  casein.1 

Conditions  determining:  quality.  In  determining  the  qual- 
ity of  cheese,  several  factors  are  to  be  taken  into  considera- 
tion. First  and  foremost  is  the  flavor,  which  determines 
more  than  anything  else  the  value  of  the  product.  This 
should  be  mild  and  pleasant,  although  with  age  the  inten- 
sity of  the  same  generally  increases  but  at  no  time  should 
it  have  any  bitter,  sour,  or  otherwise  undesirable  taste  or 


FIG.  32.    Showing  texture  of  cheese  cured  at  different  temperatures.    Right 
hand,  60°  F.,  crumbly  texture;  left  hand,  40°  F.,  waxy  texture. 

» 17  Kept.  Wis.  Expt.  Stat,,  1900,  p.  103. 
13 


178  Dairy  Bacteriology. 

aroma.  Texture  registers  more  accurately  the  physical 
nature  of  the  ripening.  The  cheese  should  not  be  curdy 
and  harsh,  but  should  yield  quite  readily  to  pressure  under 
the  thumb,  becoming  on  manipulation  waxy  and  plastic 
instead  of  crumbly  or  mealy.  Body  refers  to  the  openness 
or  closeness  of  the  curd  particles,  a  close,  compact  mass 
being  most  desirable.  The  color  of  cheese  should  be  even, 
not  wavy,  streaked  or  bleached. 

For  a  cheese  to  possess  all  of  these  characteristics  in  an 
optimum  degree  is  to  be  perfect  in  every  respect  —  a  condi- 
tion that  is  rarely  reached. 

So  many  factors  influence  this  condition  that  the  problem 
of  making  a  perfect  cheese  becomes  exceedingly  difficult. 
Not  only  must  the  quality  of  the  milk  —  the  raw  material 
to  be  used  in  the  manufacture — be  perfectly  satisfactory, 
but  the  factory  management  while  the  curds  are  in  the  vat 
demands  great  skill  and  careful  attention;  and  finally,  the 
long  period  of  curing  in  which  variation  in  temperature 
or  moisture  conditions  may  seriously  affect  the  quality, — 
all  of  these  stages,  more  or  less  critical,  must  be  successfully 
gone  through,  before  the  product  reaches  its  highest  state 
of  development. 

It  is  of  course  true  that  many  phases  of  this  complex 
series  of  processes  have  no  direct  relation  to  bacteria,  yet 
it  frequently  happens  that  the  result  attained  is  influ- 
enced at  some  preceding  stage  by  the  action  of  bac- 
teria in  one  way  or  another.  Thus  the  influence  of  the 
acidity  developed  in  the  curds  is  felt  throughout  the  whole 
life  of  the  cheese,  an  over-development  of  lactic-acid  bac- 
teria producing  a  sour  condition  that  leaves  its  impress  not 
only  on  flavor  but  texture.  An  insufficient  development 
of  acid  fails  to  soften  the  curd-particles  so  as  to  permit  of 


Bacteria  in  Cheese.  179 

close  matting,  the  consequence  being  that  the  body  of  the 
cheese  remains  loose  and  open,  a  condition  favorable  to  the 
development  of  gas-generating  organisms. 

Production  Of  flavor.  The  importance  of  flavor  as  deter- 
mining the  quality  of  cheese  makes  it  imperative  that  the 
nature  of  the  substances  that  confer  on  cheese  its  peculiar 
aromatic  qualities  and  taste  be  thoroughly  understood.  .It 
is  to  be  regretted  that  the  results  obtained  so  far  are  not 
more  satisfactory,  for  improvement  in  technique  is  hardly 
to  be  expected  until  the  reason  for  the  process  is  thoroughly 
understood. 

The  view  that  is  most  generally  accepted  is  that  this 
most  important  phase  of  cheese  curing  is  dependent  upon 
bacterial  activity,  but  the  organisms  that  are  concerned  in 
this  process  have  not  as  yet  been  satisfactorily  determined. 
In  a  number  of  cases,  different  species  of  bacteria  have 
been  separated  from  milk  and  cheese  that  have  the  power 
of  producing  aromatic  compounds  that  resemble,  in  some 
cases,  the  peculiar  flavors  and  odors  that  characterize  some 
of  the  foreign  kinds  of  cheese;  but  an  introduction  of  these 
into  curd  has  not  resulted  in  the  production  of  the  peculiar 
variety,  even  though  the  methods  of  manufacture  and  cur- 
ing were  closely  followed.  The  similarity  in  germ  content 
in  different  varieties  of  cheese  made  in  the  same  locality 
has  perhaps  a  bearing  on  this  question  of  flavor  as  related 
to  bacteria.  Of  the  nine  different  species  of  bacteria  found 
in  Emmenthaler  cheese  by  Adametz,  eight  of  them  were 
also  present  in  ripened  Hauskase.  If  specific  flavors  are 
solely  the  result  of  specific  bacterial  action,  it  might  natu- 
rally be  expected  that  the  character  of  the  flora  would  differ. 

Some  suggestive  experiments  were  made  by  Babcockand 
Russell  on  the  question  of  flavor  as  related  to  bacterial 


ISO  Dairy  Bacteriology. 

growth,  by  changing  the  nature  of  the  environment  in 
cheese  by  washing  the  curds  on  the  racks  with  warm 
water.  In  this  way  the  sugar  and  most  of  the  ash  were  re- 
moved. Under  such  conditions  the  character  of  the  bac- 
terial flora  was  materially  modified.  While  the  liquefying 
type  of  bacteria  was  very  sparse  in  normal  cheddar,  they 
developed  luxuriantly  in  the  washed  cheese.  The  flavor 
at  the  same  time  was  markedly  affected.  The  control  ched- 
dar was  of  good  quality,  while  that  made  from  the  washed 
curds  was  decidedly  off,  and  in  the  course  of  ripening  be- 
came vile.  It  may  be  these  two  results  are  simply  coinci- 
dences, but  other  data1  bear  out  the  view  that  the  flavor 
was  to  some  extent  related  to  the  nature  of  the  bacteria 
developing  in  the  cheese.  This  was  strengthened  materi- 
ally by  adding  different  sugars  to  washed  curds,  in  which 
case  it  was  found  that  the  flavor  was  much  improved,  while 
the  more  normal  lactic-acid  type  of  bacteria  again  became 
predominant. 

Ripening  Of  moldy  Cheese.  In  a  number  of  foreign 
cheeses,  the  peculiar  flavor  obtained  is  in  part  due  to  the 
action  of  various  fungi  which  grow  in  the  cheese,  and  there 
produce  certain  by-products  that  flavor  the  cheese.  Among 
the  most  important  of  these  are  the  Roquefort  cheese  of 
France,  Stilton  of  England,  and  Gorgonzola  of  Italy. 

Roquefort  cheese  is  made  from  goat's  or  cow's  milk,  and 
in  order  to  introduce  the  desired  mold,  which  is  the  ordi- 
nary bread-mold,  Penicillium  glaucum,  carefully-prepared 
moldy  bread-crumbs  are  added  to  the  curd. 

At  ordinary  temperatures  this  organism  develops  too 
rapidly,  so  that  the  cheese  to  ripen  properly  must  be  kept 
at  a  low  temperature.  The  town  of  Roquefort  is  situated 

i  Bibcock  and  Russell,  18  Kept.  Wis.  Expt.  Stat.,  1901. 


Bacteria  in  Cheese.  181 

in  a  limestone  country,  in  a  region  full  of  caves,  and  it  is 
in  these  natural  caves  that  most  of  the  ripening  is  done. 
These  caverns  are  always  very  moist  and  have  a  tempera- 
ture ranging  from  35°  to  44°  F.,  so  that  the  growth  of  the 
fungus  is  retarded  considerably.  The  spread  of  the  mold 
throughout  the  ripening  mass  is  also  assisted  in  a  mechan- 
ical way.  The  partially-matured  cheese  are  run  through 
a  machine  that  pricks  them  full  of  small  holes.  These 
slender  canals  allow  the  mold  organism  to  penetrate  the 
whole  mass  more  thoroughly,  the  moldy  straw  matting 
upon  which  the  ripening  cheese  are  placed  helping  to  fur- 
nish an  abundant  seeding  of  the  desired  germ. 

When  new  factories  are  constructed  it  is  of  advantage 
to  introduce  this  necessary  germ  in  quantities,  and  the 
practice  is  sometimes  followed  of  rubbing  the  walls  and 
cellars  of  the  new  location  with  material  taken  from  the 
old  established  factory.  In  this  custom,  developed  in  purely 
an  empirical  manner,  is  to  be  seen  a  striking  illustration 
of  a  bacteriological  process  crudely  carried  out. 

In  the  Stilton  cheese,  one  of  the  highly  prized  moldy 
cheeses  of  England,  the  desired  mold  fungus  is  introduced 
into  the  green  cheese  by  exchanging  plugs  taken  with  a 
cheese  trier  from  a  ripe  Stilton. 

Ripening  of  soft  cheese.  The  type  of  ripening  which 
takes  place  in  the  soft  cheeses  is  materially  different  from 
that  which  occurs  in  the  hard  type.  In  many  cases,  the 
peptoiiizing  action  does  not  go  on  uniformly  through- 
out the  cheese,  but  is  hastened  on  the  exterior  by  the  de- 
velopment of  organisms  that  exert  a  solvent  effect  on  the 
casein.  For  this  reason,  soft  cheeses  are  usually  made  up 
in  small  sizes,  so  that  this  action  may  be  facilitated.  The 
bacteria  that  take  part  in  this  process  are  those  that  are 


182  Dairy  Bacteriology. 

able  to  form  enzyms  (similar  in  their  action  to  tr37psinr 
galactase,  etc.),  and  these  soluble  ferments  gradually  diffuse 
from  the  outside  through  the  cheese. 

Most  of  these  peptonizing  bacteria  are  hindered  in  their 
growth  by  the  presence  of  lactic  acid,  so  that  in  many  cases 
the  appearance  of  the  digesting  organisms  on  the  surface 
is  delayed  until  the  acidity  of  the  mass  is  reduced  to  the 
proper  point  by  the  development  of  other  organisms,  prin- 
cipally molds,  which  prefer  an  acid  substratum  for  their 
growth. 

In  Brie  cheese  a  blue  coating  of  mold  develops  on  the 
surface.  In  the  course  of  a  few  weeks,  a  white  felting  ap- 
pears which  later  changes  to  red.  This  slimy  coat  below 
the  mold  layer  is  made  up  of  diverse  species  of  bacteria  and 
fungi  that  are  able  to  grow  after  the  acid  is  reduced  by  the 
blue  mold.  The  organisms  in  the  red  slimy  coat  act  upon 
the  casein,  producing  an  alkaline  reaction  that  is  unfavor- 
able to  the  growth  of  the  blue  mold.  Two  sets  of  organ- 
isms are,  therefore  essential  in  the  ripening  process,  one 
preparing  the  soil  for  the  ferment  that  later  produces  the 
requisite  ripening  changes.  As  ordinarily  carried  on,  the 
process  is  an  empirical  one,  and  if  the  red  coat  does  not  de- 
velop as  expected,  the  maker  resorts  to  all  kinds  of  devices 
to  bring  out  the  desired  ferment.  The  appearance  of  the 
right  form  is  dependent,  however,  upon  the  proper  reaction 
of  the  cheese,  and  if  this  is  not  suitable,  the  wished-for 
growth  will  not  appear. 

INFLUENCE  OF  BACTERIA  IN  ABNORMAL  CHEESE  PROCESSES. 

The  reason  why  cheese  is  more  subject  to  abnormal  fer- 
mentation than  butter  is  because  its  high  nitrogen  content 
favors  the  continued  development  of  bacteria  for  some  time 


Bacteria  in  Cheese.  183 

after  it  is  made.  It  must  be  borne  in  mind,  in  considering 
the  more  important  of  these  changes,  that  not  all  defective 
conditions  in  cheese  are  attributable  to  the  influence  of 
living  organisms.  Troubles  frequently  arise  from  errors  in 
manufacturing  details,  as  too  prolonged  cooking  of  curds, 
too  high  heating,  or  the  development  of  insufficient  or  too 
much  acid.  Then  again,  the  production  of  undesirable 
flavors  or  impairment  in  texture  may  arise  from  imperfect 
curing  conditions. 

Our  knowledge  regarding  the  exact  nature  of  these  indefi- 
nite faults  is  as  yet  too  inadequate  to  enable  many  of  these 
undesirable  conditions  to  be  traced  to  their  proper  source; 
but  in  many  cases  the  taints  observed  in  a  factory  are  due 
to  the  abnormal  development  of  certain  bacteria,  capable 
of  evolving  unpleasant  or  even  putrid  odors.  Most  of  them 
are  seeded  in  the  milk  before  it  comes  to  the  factory  and 
are  due  to  careless  manipulation  of  the  milk  while  it  is  still 
on  the  farm.  Others  gain  access  to  the  milk  in  the  fac- 
tory, owing  to  unclean  conditions  of  one  sort  or  another. 
Sometimes  the  cheese-maker  is  able  to  overcome  these 
taints  by  vigorous  treatment,  but  often  they  pass  on  into 
the  cheese,  only  to  detract  from  the  market  value  of  the 
product.  Most  frequently  these  "off"  flavors  appear  in 
cheese  that  are  cured  at  too  high  temperatures,  say  above 
65°  F. 

"Gassy"  fermentations  in  cheese.  One  of  the  worst  and 
at  the  same  time  most  common  troubles  in  cheese-making 
is  where  the  cheese  undergoes  a  fermentation  marked  by 
the  evolution  of  gas.  The  presence  of  gas  is  recognized  by 
the  appearance  either  of  spherical  or  lens-shaped  holes  of 
various  sizes  in  the  green  cheese;  often  they  appear  in 
the  curd  before  it  is  put  to  press.  Usually  in  this  condi- 


184 


Dairy  Bacteriology. 


tion  the  curds  look  as  if  they  had  been  punctured  with  a 
pin,  and  are  known  as  upin  holey"  curds.  Where  the  gas 
holes  are  larger,  they  are  known  as  "Swiss  holes"  from 
their  resemblance  to  the  normal  holes  in  the  Swiss  pro- 
duct. If  the  development  of  gas  is  abundant,  these  holes 
are  restricted  in  size.  Often  the  formation  of  gas  may  be 
so  intense  as  to  cause  the  curds  to  float  on  the  surface  of 
the  whey  before  they  are  removed.  Such  curds  are  known 
as  "floaters"  or  "bloatars." 

If  "  gassy  "  curds  are  put  to  press,  the  abnormal  fermen- 
tation may  continue.  The  further  production  of  gas  causes 
the  green  cheese  to  "  huff"  or  swell,  until  it  may  be  con- 
siderably distorted  as  in  Fig.  33.  In  such  cases  the  texture 


Fio.  33.    Cheese  made  from  gassy  milk. 

of  the  cheese  is  greatly  injured,  and  the  flavor  is  generally 
impaired. 

Such  abnormal  changes  may  occur  at  any  season  of  the 
year,  but  the  trouble  is  most  common  in  summer,  espe- 
cially in  the  latter  part. 

This  defect  is  less  likely  to  occur  in  cheese  that  is  well 


Bacteria  in  Cheese.  185 

cheddared  than  in  sweet  curd  cheese.  When  acidity  is  pro- 
duced, these  gassy  fermentations  ar^  checked,  and  in  good 
cheddar  the  body  is  so  close  and  firm  as  not  readily  to  per- 
mit of  gaseous  changes. 

In  Swiss  cheese,  which  is  essentially  a  sweet  curd  cheese, 
these  fermentations  are  very  troublesome.  Where  large 
holes  are  formed  in  abundance  (blahen),  the  trouble  reaches 
its  maximum.  If  the  gas  holes  are  very  numerous  and 


FIG.  34.    Block  Swiss  cheese  showing  "  gassy  "  fermentation.    ' 

therefore  small  it  is  called  a  "  nissler."  Sometimes  the 
normal  "  eyes "  are  even  wanting  when  it  is  said  to  be 
itblind"ora"glasler." 

One  method  of  procedure  which  is  likely  to  cause  trouble 
in  Swiss  factories  is  often  produced  by  the  use  of  sour, 
fermented  whey  in  which  to  soak  the  natural  rennets. 
Freudenreich  and  Steinegger J  have  shown  that  a  much  more 
uniform  quality  of  cheese  can  be  made  with  rennet  extract 
if  it  is  prepared  with  a  starter  made  from  a  pure  lactic  fer- 
ment. 

The  cause  of  the  difficulty  has  long  been  charged  to  va- 

i  Cent.  f.  Bakt.  1899,  p.  14. 


186  Dairy  Bacteriology. 

rious  sources,  such  as  a  lack  of  aeration,  improper  feeding, 
retention  of  animal  gases,  etc.,  but  in  all  these  cases  it  was 
nothing  more  than  a  surmise.  Very  often  the  milk  does 
not  betray  any  visible  symptom  of  fermentation  when  re- 
ceived, and  the  trouble  is  not  to  be  recognized  until  the 
process  of  cheese-making  is  well  advanced. 

Studies  from  a  biological  standpoint  have,  howevert 
thrown  much  light  on  this  troublesome  problem;  and  it 
is  now  known  that  the  formation  of  gas,  either  in  the 
curd  or  after  it  has  been  put  to  press,  is  due  entirely 
to  the  breaking  down  of  certain  elements,  such  as  the 
sugar  of  milk,  due  to  the  influence  of  various  living  germs. 
This  trouble  is,  then,  a  type  fermentation,  and  is,  therefore, 
much  more  widely  distributed  than  it  would  be  if  it  was 
caused  by  a  single  specific  organism.  These  gas-produc- 
ing organisms  are  to  be  found,  sparingly  at  least,  in  al- 
most all  milks,  but  are  normally  held  in  check  by  the 
ordinary  lactic  species.  Among  them  are  a  large  number 
of  the  bacteria,  although  yeasts  and  allied  germs  are  often 
present  and  are  likewise  able  to  set  up  fermentative  changes 
of  this  sort.  In  these  cases  the  milk-sugar  is  decomposed 
in  such  a  way  as  to  give  off  CO  9  and  H,  and  in  some  casesr 
alcohol. 

According  to  Guillebeau,  a  close  relation  exists  between 
those  germs  that  are  able  to  produce  an  infectious  inflam- 
mation (mastitis)  in  the  udder  of  the  cow  and  some  forms 
capable  of  gas  evolution.  Several  outbreaks  of  "  gassy  " 
milk  have  been  traced  directly  to  animals  suffering  from 
an  acute  udder  inflammation  in  which  it  has  been  shown 
that  the  organisms  producing  this  disease  were  the  direct 
cause  of  the  gas  production  in  the  milk. 

If  pure  cultures  of  these  gas-producing  bacteria  are  added 


Bacteria  in  Cheese.  187 

to  perfectly  sweet  milk,  it  is  possible  to  artificially  produce 
the  conditions  in  cheese  that  so  frequently  appear  in  prac- 
tice. 

Treatment  of  "  pin-holey  "  curds.  When  this  type  of 
fermentation  appears  during  the  manufacture  of  the  cheese, 
the  maker  can  control  it  in  part  within  certain  limits. 
These  methods  of  treatment  are,  as  a  rule,  purely  mechan- 
ical, as  when  the  curds  are  piled  and  turned,  and  subse- 
quently ground  in  a  curd  mill.  After  the  gas  has  been 
forced  out,  the  curds  are  then  put  to  press  and  the  whole 
mats  into  a  compact  mass. 

Another  method  of  treatment  based  upon  bacteriological 
principles  is  the  addition  of  a  starter  to  induce  the  for- 
mation of  acid.  Where  acid  is  developed  as  a  result  of 
the  growth  of  the  lactic-acid  bacteria,  the  gas-producing 
species  do  not  readily  thrive.  Another  reason  why  acid 
aids  in  repressing  the  development  of  gas  is  that  the  curd 
particles  are  partially  softened  or  digested  by  the  action  of 
the  acid.  This  causes  them  to  mat  together  more  closely, 
and  there  is  not  left  in  the  cheese  the  irregular  mechan- 
ical openings  in  which  the  developing  gas  may  find  lodg- 
ment. 

Another  method  that  is  also  useful  with  these  curds  is  to 
employ  salt.  This  represses  gaseous  fermentations,  and  the 
use  of  more  salt  than  usual  in  making  the  cheese  will  very 
often  restrain  the  production  of  gas.  Tendency  to  form 
gas  in  Edam  cheese  is  controlled  by  the  addition  of  a  starter 
prepared  from  slimy  whey  (lange  wei)  which  is  caused  by 
the  development  of  an  acid-forming  organism. 

Some  have  recommended  the  custom  of  washing  the 
curds  to  remove  the  whey  and  the  gas-producing  bacteria 
contained  therein.  Care  must  be  taken  not  to  carry  this 


188  Dairy  Bacteriology. 

too  far,  for  the  removal  of  the  sugar  permits  taint-produc- 
ing organisms  to  thrive.1 

The  temperature  at  which  the  cheese  is  cured  also  mate- 
rially affects  the  development  of  gas.  At  high  curing  tem- 
peratures, gas-producing  organisms  develop  rapidly;  there- 
fore more  trouble  is  experienced  in  summer  than  at  other 
seasons. 

If  milks  which  are  prone  to  undergo  "  gassy  "  develop- 
ment are  excluded  from  the  general  supply,  it  would  be 
possible  to  eliminate  the  source  of  the  entire  trouble.  To 
aid  in  the  early  recognition  of  such  milks  that  are  not  ap- 
parently affected  when  brought  to  the  factory,  fermenta- 
tion or  curd  tests  (p.  76)  are  of  great  value.  The  use  of 
this  test  in  the  hands  of  the  factory  operator  often  enables 
him  to  detect  the  exact  source  of  the  trouble,  which  may 
frequently  be  confined  to  the  milk  delivered  by  a  single 
patron. 

"Fruity"  or  "sweet"  flavor.  Not  infrequently  the 
product  of  a  factory  may  acquire  during  the  process  of 
ripening  what  is  known  as  a  "  sweet"  or  u fruity"  flavor. 
This  flavor  resembles  the  odor  of  fermented  fruit  or  the 
bouquet  of  certain  kinds  of  wine.  It  has  been  noted  in 
widely  different  sections  of  the  country  and  its  presence 
bears  no  relation  to  the  other  qualities  of  the  cheese.  The 
cause  of  this  trouble  has  recently  been  traced2  to  the  pres- 
ence of  various  kinds  of  yeasts.  Ordinarily  yeasts  are 
rarely  present  in  good  cheese,  but  in  cheese  affected  with 
this  trouble  they  abound.  The  addition  of  starters  made 
from  yeast  cultures  resulted  in  the  production  of  the  unde- 
sirable condition. 

1  Babcock  and  Russell,  18  Kept.  Wis.  Expt.  Stat.,  1901. 

3  Harding,  Rogers  and  Smith,  Bull.  183,  N.  Y.  (Geneva)  Expt.  Stat.,  Dec.,  1900. 


Bacteria  in  Cheese.  189 

Mottled  Cheese.  The  color  of  cheese  is  sometimes  cut  to 
that  extent  that  the  cheese  presents  a  wavy  or  mottled  ap- 
pearance. This  condition  is  apt  to  appear  if  the  ripening 
temperature  is  somewhat  high,  or  larger  quantities  of  ren- 
net used  than  usual.  The  cause  of  the  defect  is  obscure, 
but  it  has  been  demonstrated  that  the  same  is  communi- 
cable if  a  starter  is  made  by  grating  some  of  this  mottled 
cheese  into  milk.  The  bacteriology  of  the  trouble  has  not 
yet  been  worked  out,  but  the  defect  is  undoubtedly  due  to 
an  organism  that  is  able  to  grow  in  the  ripening  cheese. 
It  has  been  claimed  that  the  use  of  a  pure  lactic  ferment 
as  a  starter  enables  one  to  overcome  this  defect. 

Bitter  Cheese.  Bitter  flavors  are  sometimes  developed  in 
cheese  especially  where  the  ripening  process  has  not  been 
fully  completed,  or  where  improper  temperatures  have  been 
maintained  for  a  considerable  length  of  time.  Several  or- 
ganisms associated  with  this  abnormal  fermentation  have 
been  noted. 

Guillebeau1  isolated  several  forms  from  Emmenthaler 
cheese  which  he  connected  with  udder  inflammation  that 
were  able  to  produce  a  bitter  substance  in  cheese. 

Von  Freudenreich 2  has  described  a  new  form  Micrococ- 
cus  easel  amari  (micrococcus  of  bitter  cheese)  that  was 
found  in  a  sample  of  bitter  cheese.  This  germ  is  closely 
related  to  Conn's  micrococcus  of  bitter  milk.  It  develops 
lactic  acid  rapidly,  coagulating  the  milk  and  producing  an 
intensely  bitter  taste  in  the  course  of  one  to  three  days. 
When  milk  infected  with  this  organism  is  made  into 
cheese,  there  is  formed  in  a  few  days  a  decomposition  prod- 
uct that  imparts  a  marked  bitter  flavor  to  the  cheese. 

1  Guillebeau,  Landw.  Jahr.,  1890,  p.  27. 

2  Freudenreich,  Fuehl.  Landw.  Ztg.,  43:361. 


190  Dairy  Bacteriology. 

It  is  peculiar  that  some  of  the  organisms  that  are  able  to 
produce  hitter  products  in  milk  do  not  retain  this  property 
when  the  milk  is  worked  up  into  cheese. 

Putrid  or  rotten  Cheese.  Sometimes  cheese  undergoes 
a  putrefactive  decomposition  in  which  the  texture  is  pro- 
foundly modified  and  various  foul  smelling  gases  are 
evolved.  These  often  hegin  on  the  exterior  as  small  cir- 
cumscrihed  spots  that  slowly  extend  into  the  cheese,  chang- 
ing the  casein  into  a  soft  slimy  mass.  Then,  again,  the 
interior  of  the  cheese  undergoes  this  slimy  decomposition. 
The  soft  varieties  are  more  prone  toward  this  fermentation 
than  the  hard,  although  the  firm  cheeses  are  by  no  means 
exempt  from  the  trouble.  The  "  Verlaufen  "  or  "  running" 
of  lim burger  cheese  is  a  fermentation  allied  to  this.  It  is 
where  the  inside  of  the  cheese  breaks  down  into  a  soft 
semi-fluid  mass.  In  severe  cases,  the  rind  may  even  be 
ruptured,  in  which  case  the  whole  interior  of  the  cheese 
flows  out  as  a  thick  slimy  mass,  having  sometimes  a  putrid 
odor.  The  conditions  favoring  this  putrid  decomposition 
are  usually  associated  with  an  excess  of  moisture,  and  an 
abnormally  low  ripening  temperature. 

Rusty  Spot.  This  name  is  applied  to  the  development 
of  small  yellowish-red  or  orange  spots  that  are  formed 
sometimes  throughout  the  whole  mass  of  cheddar  cheese. 
A  close  inspection  shows  the  colored  points  to  be  located 
along  the  edges  of  the  curd  particles.  According  to  Hard- 
ing,1 this  trouble  is  most  common  in  spring  and  fall.  The 
cause  of  the  difficulty  has  been  traced  by  Connell 8  to  the 
development  of  a  chromogenic  bacterium,  Bacillus  ruden- 
sis.  The  organism  can  be  most  readily  isolated  on  a  po- 

i  Bull.  183,  N.  Y.  (Geneva)  Expt.  Stat.,  Dec.  1900. 
'Connell,  Bull.  Canadian  Dept.  of  Agr.,  1897. 


Bacteria  in  Cheese.  191 

tato  surface  rather  than  with  the  usual  isolating  media, 
agar  or  gelatin. 

Other  pigment  Changes.  Occasionally,  with  the  hard 
type  of  cheese,  but  more  frequently  with  the  softer  foreign 
varieties,  various  abnormal  conditions  arise  that  are  marked 
by  the  production  of  different  figments  in  or  on  the  cheese. 
More  frequently  these  are  merely  superficial  and  affect  only 
the  outer  layers  of  the  cheese.  Generally  they  are  attrib- 
utable to  the  development  of  certain  chromogenic  organ- 
isms (bacteria,  molds  and  yeasts),  although  occasionally 
due  to  other  causes,  as  in  the  case  of  a  blue  discoloration 
sometimes  noted  in  foreign  cheese  made  in  copper  kettles.1 

De  Vries2  has  described  a  blue  condition  that  is  found 
in  Edam  cheese.  It  appears  first  as  a  small  blue  spot  on 
the  inside,  increasing  rapidly  in  size  until  the  whole  mass 
is  affected.  This  defect  he  was  able  to  show  was  produced 
by  a  pigment-forming  organism,  B.  cyaneo-fuscus.  By  the 
use  of  slimy  whey  (lange  wei)  this  abnormal  change  was 
controlled. 

Moldy  Cheese.  With  many  varieties  of  cheese,  especially 
some  of  the  foreign  types,  the  presence  of  mold  on  the  ex- 
terior is  not  regarded  as  detrimental;  in  fact  a  limited  de- 
velopment is  much  desired.  In  hard  rennet  cheese  as 
cheddar  or  Swiss,  the  market  demands  a  product  free  from 
mold,  although  it  should  be  said  that  this  condition  is  im- 
posed by  the  desire  to  secure  a  good-looking  cheese  rather 
than  any  injury  in  flavor  that  the  mold  causes.  Mold 
spores  are  so  widely  distributed  that,  if  proper  temperature 
and  moisture  conditions  prevail,  these  spores  will  always 
develop.  At  temperatures  in  the  neighborhood  of  40°  P. 

»  SchmOger,  Milch  Zeit.,  1883,  p.  483. 
2  De  Vries,  Milch  Zeit.,  1888,  pp.  861.  885. 


192  Dairy  Bacteriology. 

and  below,  mold  growth  is  exceedingly  slow,  and  often  fructi- 
fication does  not  occur,  the  only  evidence  of  the  mold  being 
the  white,  felt-like  covering  that  is  made  up  of  the  vegetat- 
ing filaments.  The  use  of  paraffin  has  been  suggested  as  a 
means  of  overcoming  this  growth,  the  cheese  being  dipped 
at  an  early  stage  into  melted  paraffin.  Recent  experiments 
have  shown  that  "off"  flavors  are  apt  to  develop  where 
cheese  are  paraffined  directly  from  the  press.  Furthermore, 
the  paraffin  has  a  tendency  to  crack  and  separate  from  the 
rind,  thus  allowing  molds  to  develop  beneath  the  paraffin 
coat,  where  the  conditions  are  ideal  as  to  moisture,  for 
evaporation  is  excluded  and  the  air  consequently  saturated. 
The  use  of  formalin  (2$  solution)  has  been  suggested  as  a 
wash  for  the  outside  of  the  cheese.  This  substance  or 
sulfur  is  also  applied  in  a  gaseous  form.  Double  bandag- 
ing is  also  resorted  to  as  a  means  of  making  the  cheese 
more  presentable  through  the  removal  of  the  outer  bandage. 
The  nature  of  these  molds  has  not  been  thoroughly 
studied  as  yet.  •  The  ordinary  blue-green  bread  mold, 
Penicillium  glaucum,  is  most  frequently  found,  but  there 
are  numerous  other  forms  that  appear,  especially  at  low 
temperatures. 

Poisonous  Cheese*  Cases  of  acute  poisoning  arising  from 
the  ingestion  of  cheese  are  reported  from  time  to  time. 
Vaughan  has  succeeded  in  showing  that  this  condition  is 
due  to  the  formation  of  a  highly  poisonous  alkaloid  which 
he  has  isolated,  and  which  he  calls  tyrotoxicon.1  This  poison- 
ous ptomaine  has  also  been  demonstrated  in  milk  and  other 
milk  products,  and  is  undoubtedly  due  to  the  development 
of  various  putrefactive  bacteria  that  find  their  way  into 
the  milk.  It  seems  quite  probable  that  the  development 

»Zeit.  f.  physiol.  theinie,  10:146. 


Bacteria  in  Cheese.  193 

of  these  toxic  organisms  can  also  go  on  in  the  cheese  after 
it  is  taken  from  the  press. 

Prevention  or  cheese  .defects*  The  defective  conditions 
previously  referred  to  can  rarely  be  overcome  in  cheese 
so  as  to  improve  the  affected  product,  for  they  only  be- 
come manifest  in  most  cases  during  the  later  stages  of  the 
curing  process.  The  only  remedy  against  future  loss  is  to 
recognize  the  conditions  that  are  apt  to  prevail  during  the 
occurrence  of  an  outbreak  and  see  that  the  cheese  are  han- 
dled in  such  a  way  as  to  prevent  a  recurrence  of  the  diffi- 
culty. 

Many  abnormal  and  undesirable  results  are  incident  to 
the  manufacture  of  the  product,  such  as  u  sour "  or 
41  mealy  "  cheese,  conditions  due  to  the  development  of  too 
much  acid  in  the  milk  or  too  high  a  "  cook."  These  are 
under  the  direct  control  of  the  maker  and  for  them  he 
alone  is  responsible.  The  development  of  taints  due  to 
the  growth  of  unwelcome  bacteria  that  have  gained  access 
to  the  milk  while  it  is  yet  on  the  farm  are  generally  be- 
yond the  control  of  the  cheese  maker,  unless  they  are  so 
pronounced  as  to  appear  during  the  handling  of  the  curds. 
If  this  does  occur  he  is  sometimes  able,  through  the  inter- 
vention of  a  starter  or  by  varying  some  detail  in  making, 
to  handle  the  milk  in  such  a  way  as  to  minimize  the  trou- 
ble, but  rarely  is  he  able  to  eliminate  it  entirely. 

One  of  the  most  strenuous  duties  which  the  maker  must 
perform  at  all  times  is  to  point  out  to  his  patrons  the  ab- 
solute necessity  of  their  handling  the  milk  in  such  a  way 
as  to  prevent  the  introduction  of  organisms  of  a  baleful 
type. 

13 


INDEX. 


Acid,  effect  of,  on  churning,  137;  in 
butter-making,  138. 

Acid  test,  51. 

Aeration  of  milk,  59. 

Aerobic  bacteria,  7. 

Alcoholic  fermentation  in  milk,  72. 

Anaerobic  bacteria,  7. 

Animal,  influence  of,  on  milk  infec- 
tion, 42. 

Animal  odor,  56. 

Anthrax,  94. 

Antiseptics,  9,  88. 

Aroma,  of  butter,  140. 

Bacillus:  definition  of,  2. 

acidi  lactici,  64;  cyaneo-fuscus, 
188;  cyanogenus,  74;  foetidus  lactis, 
157;  lactis  aerogenes,  65;  lactis 
erythrogenes,  74;  lactis  saponacei, 
67;  lactis  viscosus,  71;  nobilis,  162, 
174;  prodigiosus,  74;  rudensis,  188; 
synxanthus,  75 ;  tuberculosis,  84. 

Bacteria:  on  hairs,  34;  kinds  in  milk,  64; 
in  barn  air,  42;  in  milk  pails,  27;  in 
butter,  154;  classification  of,  4;  in 
cheese,  160;  culture  of,  17;  in  cream, 
128;  discovery  of,  1;  external  con- 
ditions affecting,  8;  form  of,  2;  in 
butter,  142;  in  butter- making,  127; 
in  centrifuge  slime,  40;  in  fore 
milk,  30;  in  rennet,  163;  in  sepa- 
rator slime,  40;  manure,  33;  num- 
ber of,  in  milk,  49. 

Distribution    of:    milk  of  American 
cities,  49;  European  cities,  49;  in  re- 
lation to  cheese,  168. 
Of  disease:  anthrax,  94;  cholera,  98; 
diphtheria,  99;  lockjaw,  94;  toxic, 


100;  tuberculosis,  84;  typhoid  fever, 
98. 

Methods  of  study  of:    culture,  15; 
culture  media,  13;  isolation,  14. 

Bitter  butter,  158;  cheese,  189;  milk,  72. 

Bloody  milk,  74. 

Blue  cheese,  191;  milk,  74. 

Bovine  tuberculosis,  84. 

Brie  cheese,  182. 

Butter:  bacteria  in,  154;  bitter,  158; 
"cowy,"  157;  fishy,  159;  lardy, 
157;  moldy,  158;  mottled,  156;  oily, 
158;  putrid,  156;  rancid,  155;  tal- 
lowy, 157;  turnip  flavor  in,  157. 
Making:  aroma,  140;  flavor  in,  140; 
pure  culture,  143;  in  ripening  of 
cream,  136. 

Butyric  acid  fermentation,  69. 

By-products  of  factory,  methods  of  pre- 
serving, 25. 

Casease,  68 

Caseone,  68. 

Centrifugal  force,  cleaning  milk  by, 
39,  105. 

Cheese:  bacterial  flora  of,  168:  bitter, 
189;  blue,  187;  Brie,  182;  Edam,  72, 
162;  Emmen thaler,  185;  flavor  of, 
179;  gassy  fermentations  in,  183; 
Gorgonzola,  180;  molds  on,  191; 
mottled,  189;  "nissler,"  185;  poi- 
sonous, 192;  putrid,  190;  ripening 
of  moldy,  180;  ripening  of  soft,  181; 
Koquefort,  180;  rusty  spot  in,  188; 
Stilton,  180;  Swiss,  185. 
Making  and  curing:  chemical  changes 
in  curing,  166;  influence  of  temper- 
ature on  curing,  169;  influence  of 


196 


Index. 


rennet,  177;  physical  changes  in 
curing,  165;  prevention  of  defects, 
193;  starters  in,  161;  temperature 
in  relation  to  bacterial  influence, 
169. 

Theories  of  curing:    digestive,  173; 
galactase,  175,  177;  lactic  acid,  174 

Chemical  changes  in  cheese-ripening, 
166. 

Chemical  disinfectants  in  milk:  bleach- 
ing powder,  81;  corrosive  subli- 
mate, 81;  formalin,  80;  sulfur,  80; 
whitewash,  81 ;  vitriol,  81. 

Chemical  preservatives,  80. 

Children,  milk  for,  45. 

Cholera  in  milk,  98. 

Coccus,  definition  of,  2. 

Cold,  influence  on  bacteria,  8,  47. 

Contamination  of  milk  through  disease 
germs,  95, 191. 

Coolers,  181. 

Cooling  milk,  123. 

Cream,  bacterial  changes  in,  135;  me- 
chanical causes  for  bacteria  in,  135; 
pasteurized,  118;  restoration  of  con- 
sistency of  pasteurized,  119. 
Ripening  of,  136;  advantage  of  pure 
cultures  in,  144;  by  natural  start- 
ers, 142;  characteristics  of  pure 
cultures  in,  145;  objections  to  pure 
cultures  in,  146;  principles  of  pure 
cultures  in,  143;  propagation  of 
pure  cultures,  151;  home-made 
starters  in,  146. 

Creaming  methods,  134. 

Curd  test,  76. 

Dairy  utensils  a  source  of  contamina- 
tion, 21. 

Diarrhoeal  diseases,  100. 
Digesting  bacteria,  67. 
Digestibility  of  heated  milk,  lit 
Diphtheria,  99. 


Dirt  in  milk,  34. 

Dirt,  exclusion  of,  36. 

Disease  germs  in  milk,  95;  effect  of 
heat  on,  91 ;  origin  of,  83. 

Disinfectants,  9:  carbolic  acid,  81;  chlo- 
ride of  lime,  81;  corrosive  subli- 
mate, 81;  formalin,  80;  sulfur,  80; 
vitriol  salts,  81 ;  whitewash,  79. 

Disinfectants  in  milk:  alkaline  salts, 
106;  boracic  acid,  106;  formalin, 
106;  preservaline,  107;  salicylic  acid, 
106. 

Domestic  pasteurizing  apparatus,  125. 

Drugs,  taints  in  milk  due  to,  5(5. 

Drying,  effect  of,  8. 

Edam  cheese,  72,  162. 
Emmen thaler  cheese,  185. 
Endospores,  3. 
Enzyms,  10. 

Factory  by-products,  25;  treatment  of, 
25. 

Farrington  alkaline  tablet,  52. 

Fecal  bacteria,  effect  of,  on  butter,  35. 

Fermentation: 
In  cheese:  gassy,  183. 
In  milk:   alcoholic,    72;   bitter,   72; 
blue,  74;  butyric.  69;  digesting,  67; 
slimy,  69;   gassy,  66;    kephir,  72; 
koumiss,  72;  lactic  acid,  63;  lange- 
wei,  72;  red,  74;  ropy,  69;    slimy, 
69;  soapy,  73;  souring,  63;   sweet 
curdling,  67;  treatment  of,  75. 
Tests,  76;   Gerber's,  76;   Walther's, 
76;  Wisconsin  curd,  76. 

Filtration  of  milk,  104. 

Fishy  butter,  159. 

Flavor:  of  butter,  140;  of  cheese,  179. 

Foot  and  mouth  disease,  93. 

Fore  milk,  29. 

Formaldehyde,  80. 

Formalin,  80. 

Fruity  flavor  hi  cheese,  188. 


Index. 


197 


Galactase  in  cheese,  175, 

Gassy  fermentations:  in  cheese,  183; 
in  milk,  66;  in  Swiss  cheese,  167, 

Glasler,  185. 

Gorgonzola  cheese,  180. 

Growth  of  bacteria,  essential  condi- 
tions for,  4. 

Hair,  bacteria  on,  34. 

Heat,  influence  on  bacterial  growth,  8. 

Heated  .milk:  characteristics  of,  109; 
action  toward  rennet,  112;  body, 
110;  digestibility,  111;  fermentative 
changes,  111;  flavor,  110;  hydrogen 
peroxid  test  in,  23;  Storch's  test,  23. 

Hygienic  milk,  bacteria  in,  44. 

Infection  of  milk:   animal,  82;  dairy 
utensils,  8;  fore  milk,  29;  milker,  35. 
Isolation  of  bacteria,  methods  of,  14. 

Kephir,  72. 
Koumiss,  72. 

Lactic  acid:  fermentation  in  milk,  63; 

theory  in  cheese-curing,  1?4. 
Lange-wei,  72. 
Lardy  butter,  157,, 
Light,  action  on  bacteria,  9. 

Manure,  bacteria  in,  33. 

Methods:   of  isolation,  14;  culture,  15. 

Micrococcus  casei  amari,  187. 

Microscope,  use  of,  17. 

Milk:  a  bacterial  food  medium,  19;  bac- 
teria in,  48. 

Disease  organisms  in:  anthrax,  94; 
cholera,  98;  diphtheria,  99;  foot 
and  mouth  disease,  93 ;  poisonous, 
101;  ptomaines,  101;  scarlet  fever, 
99;  tuberculosis,  84;  typhoid  fever, 
98. 

Contamination,  21:  from  air,  40;  from 
animal  odors,  55;  dirt,  32;  distinc- 
tion between  bacterial  and  non- 


bacterial,  58;  fore  milk,  28;  infec- 
tion in  factory,  59;  milker,  35;  rel- 
ative importance  of  various  kinds, 
42;  utensils,  21. 

Milk  fermentations:  alcoholic,  72;  bit- 
ter, 72;  bloody,  74;  blue,  74;  buty- 
ric acid,  69;  gassy,  66,  167;  kephir, 
61;  koumiss,  67;  lactic  acid,  63;  red, 
72;  ropy,  69;  slimy,  69,  soapy,  74; 
souring,  63;  sweet  curdling,  67; 
tests  for,  76;  treatment  of,  70;  yel- 
low, 70. 

Milk,  heated:  action  towards  rennet, 
112;  digestibility,  111;  flavor  of,  110; 
fermentative  changes  in,  111;  hy- 
drogen peroxid  test,  110. 

Milking  machines,  influence  of,  on 
germ  content,  36. 

Milk  preservation:  chemical  agents  in, 
106;  condensation,  107;  freezing, 
108;  heat,  108;  pasteurization,  112; 
sterilization,  113. 

Milk-sugar  as  bacterial  food,  19. 

Mold,  in  butter,  158;  in  cheese,  191. 

Mottled  cheese,  189. 

"  Nissler  "  cheese,  185. 

Odors,  direct  absorption  of,  in  milk,  55. 
Oidium  lactis,  159. 
Oily  butter,  158. 

Parasitic  bacteria,  5. 

Pasteurization  of  milk:  acid  test  in, 
123;  bacteriological  study  of,  117, 
133, 149;  for  butter,  147;  for  cheese, 
162;  for  direct  use,  113;  of  skim 
milk,  25;  details  of,  112;  tempera- 
ture and  time  limit  in,  114. 

Pasteurizing  apparatus:  continuous 
flow,  127;  coolers,  131;  Danish,  149; 
domestic,  125;  intermittent  flow, 
129;  Potts,  131;  Reid,  150;  Russell, 
130. 

Pathogenic  bacteria  in  milk,  82. 


198 


Index. 


Penicillium  glaucum,  159,  180, 190. 

Pepsin,  10. 

Physical  changes  in  cheese-ripening, 
165. 

Poisonous  bacteria:  in  cheese,  192;  in 
milk,  100,  101. 

Preservaline,  167. 

Preservation  of  milk:  by  exclusion,  103; 
chemical  agents,  106;  condensing, 
107;  filtration,  104;  freezing  ,  108; 
pasteurization,  112;  physical 
agents,  107;  sterilization,  112. 

Ptomaine  poisoning,  101. 

Pure  cultures,  15. 

Pure  culture  starters:  advantages  of 
144;  characteristics  of,  145;  home- 
made cultures  compared  with,  146; 
propagation  of,  157. 

Putrid  cheese,  190;  butter,  156. 

Rancidity  in  butter,  155. 

Red  milk,  74. 

Rennet:  action  in  heated  milk,  112; 
bacteria  in,  163;  influence  of,  on 
cheese-ripening,  177. 

Restoration  of  consistency  in  pasteur- 
ized cream,  119. 

Ripening  of  cheese:  moldy  cheese,  180; 

soft  cheese,  181. 

Of  cream,  136;  artificial  starters,  143; 
natural  starters,  142;  principles  of 
pure  culture  starters  in,  143. 

Ropy  milk,  69. 

Roquefort  cheese,  180. 

Rusty  spot  in  cheese,  190. 

Rusty  cans:  effect  of,  on  acidity,  53. 

Sanitary  milk,  44. 
Sanitary  pails,  39. 
Saprophytic  bacteria,  5. 
Scarlet  fever  in  milk,  99. 
Separator  slime:  bacteria  in,  40;  tuber- 
cle bacillus  in,  93. 
Size  of  bacteria,  2. 


Skim-milk,  a  distributor  of  disease, 
96. 

Slimy  milk,  69. 

Soapy  milk,  74. 

Soft  cheese,  ripening  of,  186. 

Sources  of  contamination  in  milk:  barn 
air,  40;  dairy  utensils,  21;  dirt  from 
animals,  32;  factory  cans,  27;  fore- 
milk, 29;  milker,  35. 

Souring  of  milk,  63. 

Spirillum,  definition  of,  2. 

Spores,  3. 

Starters:  in  cheese-making,  161;  in 
butter-making,  142;  propagation 
of,  151;  pure  cultures  in  cream- 
ripening,  143. 

Sterilization  of  milk,  112. 

Streptococcus  Hollandicus,  72,  162. 

Stilton  cheese,  181. 

Starch's  test,  23. 

Sulfur  as  a  disinfectant,  81. 

Sweet  curdling  milk,  68. 

Sweet  flavor  in  cheese.  188. 

Swiss  cheese,  177;  gassy  fermentations 
in,  185. 

Taints,  absorption  of,  55. 

Taints,  bacterial  vs.  physical,  58. 

Taints  in  milk,  absorption  of,  55. 

Taints,  use  of  starters  in  overcoming, 
79. 

Taints  in  butter:  putrid,  156;  rancidity, 
155;  turnip  flavor,  157. 

Tallowy  butter,  157. 

Temperature:  effect  on  bacterial  de- 
velopment, 6,  46;  effect  of  low,  108; 
effect  of  high,  108;  and  time  limit 
in  milk  pasteurization,  114. 

Tests  for  milk:  fermentation,  76; 
Storch's,  23. 

Theories  in  cheese-curing:  digestive, 
171;  galactase,  175, 177;  lactic  acid, 
174. 

Trypsin,  10. 


Index. 


199 


Tubercle  bacillus:  in  milk,  90;  in  sep- 
arator slime,  93;  thermal  death 
limits,  115. 

Tuberculin  test,  86. 

Tuberculosis,  bovine,  84. 

Turnip  flavor  in  butter,  157. 

Typhoid  fever,  98. 

Tyrogen,  162. 

Tyrotoxicon,  101,  190. 

Udder:  milk  germ-free  in,  19;  infection 
of,  38;  washing,  37;  tuberculosis  in, 
87. 


Viscogen,  119. 

Water:  as  a  source  of  infection,  61. 
Whey  vats,  pollution  of,  23;  method  of 

preserving,   125;   treatment  of,  in 

vats,  123. 
Whitewash,  81. 
Wisconsin  curd  test,  76. 

Yeasts:  alcoholic  ferments  in  milk,  72; 
kephir,  72;  fruity  flavor  in  cheese, 
186. 


92011 


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